JANJARY, 1934 I Con&n's mclude The First Mirror -screw Recrivr for the Amateur Some ?roblems in Catl-oce-ray Television Picture Shapes and Scanning Lines A Si:np_e Disc Receiver St-alin and Screen The Taeori of the Kerr Cell Patc ats and Progress First Letai of a New S}sttm of Reception Foreign News, Answers to CDr:espondents, New Apr amt -is, etc., etc. THE FIRST SCOPHONY PICTURES Vol. VII No. 71 N SERI E A. MIRROR -SCREW RECEIVER The mirror :crew receiuel is one of the moat efficient of t1 mecharical type 7,1 produced ani it has the further advantage of simplicity. The screw is ba rp - of a numLe- of fiat -r etal plates arranged in the manner of a stint sta.rcas2. The reviver illustrated is the " Tecade. '
Transcript
JANJARY, 1934 I
Con&n's mclude
The First Mirror -screwRecrivr for the Amateur
Some ?roblems inCatl-oce-ray Television
Picture Shapes andScanning Lines
A Si:np_e Disc Receiver
St-alin and Screen
The Taeori ofthe Kerr Cell
Patc ats and ProgressFirst Letai of a New
S}sttm of ReceptionForeign News, Answersto CDr:espondents, New
Apr amt -is, etc., etc.THE FIRST SCOPHONY
PICTURES
Vol. VII No. 71
N SERI E
A. MIRROR -SCREW RECEIVERThe mirror :crew receiuel is one of the moat efficient of t1 mecharical type 7,1produced ani it has the further advantage of simplicity. The screw is ba rp-
of a numLe- of fiat -r etal plates arranged in the manner of a stint sta.rcas2.The reviver illustrated is the " Tecade. '
;a.1.:,'VIS4.1.CA ANUARY, 1934
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2
No. 71 VOL. VII .1 ANUARY, 1934.
=
THE FIRST TELEVISION JOURNAL IN THE WORLD
In This IssueConstructive criticism of the B.B.C.'s policy
of television broadcasting by Mr. S. Sagall,Managing Director of Scophony, Ltd.
' The first of a series of informative articles,by G. Parr, on the problems of cathode-raytelevision.
* * *
Full constructional details of the firstmirror -screw receiver for the amateur-acheap, simple and efficient apparatus for thehome -constructor.
* *
A practical and informative article on thetheory of the Kerr cell, by J. C. Wilson.
* * *
First and exclusive pictures of the Scophonyreceivers.
* * *
Records of latest developments, includingthe first description of a novel receivingsystem.
* * *
" Electro-optics "-a comprehensive reportof the December lecture given before theTelevision Society.
* *
Some considerations on the design of anH.F. stage in a television receiver.
* * *
" Easy -to -understand " instructions for con-necting the neon lamp to the wireless set.
TELEVISIONProprietors :
BERNARD JONES PUBLICATIONS, LTDEditor -in -Chief :
(Last Wednesday in every month forfollowing month).
Contributions are invited and will bepromptly considered. Correspondenceshould be addressed, according to itsnature, to the Editor, the AdvertisementManager, or the Publisher, " Tele-vision," 58-61, Fetter Lane, London,
E.C.4.
=
COMMENT OF THE AIONTEIOurselves.
WITH this issue, TELEVISION passes into the hands of Bernard JonesPublications, Ltd., proprietors of AMATEUR WIRELESS and WIRELESS
MAGAZINE, and thus comes under the control of publishers whose sole interestsin the past have been devoted to wireless publications and who are thereforein a unique position to direct a journal devoted to a subject which is so closelyallied to radio.
The advantage of such an association need not be stressed, but it may bepointed out that a highly trained technical staff and research facilities are nowat the disposal of this journal.
Whilst the publication of a separate magazine devoted to television is a newstep for the present publishers, there is nothing new whatever in their asso-ciation with television itself. The Editor and Staff of TELEVISION and itsassociated technical papers have been present at every outstanding demon-stration of television during the last ten years and have kept closely in touchwith the whole subject. Indeed, ever since the practicability of televisionbecame apparent, AMATEUR WIRELESS has assisted and encouraged its develop-ment and this has been a considerable factor in the determination of thepublishers to acquire this journal and thus give their energetic assistance tothe furthering of a policy which can be summed up in four words " TheDevelopment of Television." To this end our energies are pledged and ourreaders' support is invited.
The B.C.C. TransmissionsE publish below the latest official statement of the B.B.C. regarding thetelevision transmissions, and refer our readers to the excellent summing
up of the position by Mr. S. Sagall on other pages in this issue."In September notice was given to Baird Television, Ltd., of the termina-
tion on March 31, 1934, of the arrangement under which regular televisionprogrammes are transmitted on a medium wavelength using their "3o -line"system.
Meanwhile, experiniental work is being carried out with high -definitionsystems transmitted on ultra -short wavelengths. Such systems offer morepossibilities of future development than the low -definition systems, althoughonly the latter can be transmitted pn medium wavelengths. If, however,the development of high -definition television is not sufficiently stabilisedby March 31 to justify regular transmissions by any of the methods tested,then the B.B.C. may continue transmissions, probably twice a week, usingthe low -definition method on an ordinary broadcast wavelength, with a viewto assisting those members of the pubic who are experimenting in tele-vision. These transmissions would depend on future development, withno guaranteed duration."We may remark that, even assuming that the high -definition experiments
are a success from a technical point of view, it is quite improbable that theapparatus required for the reception of these transmissions will be within thereach of any considerable number of people, whereas those participating inthe reception of the 3o -line transmissions are a steadily increasing section of thelistening community, and this would immediately be enlarged were somedefinite statement of policy forthcoming.
3
faLIY1S10P1 JANUARY, 1934
Television in 9 3 4By S. Sagall
Our authoritative contributor is Managing Director of Scophony Limited, and one of the pioneers ofcommercial television. Scophony Limited is engaged on the develcpment of a novel system of sound andpicture communications, based on Mr. G. W. Walton's inventions. Secrecy still surrounds the technicalactivities of the Scophony laboratory, though some conception of the revolutionary character of the in-ventions may be gained from the photographs, thefirst ever to be released by Scophony Limited and placed at
the exclusive disposal of Television.
fr. S. Sagall, Managing Director of Scophony Ltd.
WRITING this as I do in the lastdays of 1933, the heading -maysuggest " prophisus." In
television we have had, during the lastfive or six years, far too many of them.I am accordingly dealing with " reali-ties " and actual achievements.
I feel that the time has arrived whena complete case must be made out for" 3o -line television," and if convincing,this view must be supported by the fullweight of public opinion, before it istoo late.
It is my opinion that a great stimuluswas given to the development of tele-vision in this country by the fact oftine B.B.C. establishing, some eighteenmonths ago, a regular television broad-casting service. The entertainmentvalue of the programmes offered at thecommencement of the transmissionswas very low, and it was only early thisyear that a remarkable improvementbecame apparent. The B.B.C. was thefirst-at least, as far as Europe is con-cerned-to make a proper and thoroughstudy of the whole problem of tele-
vision studio technique and its special-ised requirements. The B.B.C. justlydeserves the compliments of all thoseinterested in the art, for the part itplayed in bringing the 3o -line trans-missions to such a high standard.
ImprovedTechnique
Compare " 3o -line " pictures as seena few years ago, even those of twelvemonths ago, with what is offered now.What a vast improvement ! One wouldnever have expected that such resultscould be obtained from what is, afterall, only a 3o -line picture.
This tremendous improvement is, ofcourse, also due to the greater efficiencyof modern television receivers. The differ-ence between a 3o -line picture of a fewyears ago, the size of a postage stamp,as seen on a Nipkow disc, and a B.B.C.picture received on a Scophony 3o -linerotating echelon (size of picture 34 ins.by 84 ins.) or on a Baird mirror -drum,is like that between a doll with eyesmade to move and a strong healthyinfant.
Now, at some time or other, com-plaints were made by those " rulingthe waves " that the public shows verylittle interest in the television trans-missions. Of course, the number ofthose possessing television receiverswas bound to be small, as long as therewere no sets on sale. And, naturally,manufacturers would not take the riskof bringing out a receiver until theentertainment value of the picture wasamply proved.
A SetBack
I would submit that this stage wasreached by the end of the summer 1933.Scophony, Ltd., felt that there was agrowing demand for an adequate re-ceiver and made arrangements to placeit on the market. About the sametime, the Bush Radio receiver becameavailable. It seems, however, mostincomprehensible that just then (in
September, 1933) the B.B.C. shouldintervene by announcing the possiblediscontinuation of the 3o -line televisiontransmissions. As far as Scophony,Ltd., was concerned, all manufacturingarrangements had to be suspended. Thelatest statement by the B.B.C. leavesthe position and everybody concernedvery much in the dark, so that, ofcourse, no further progress in thedirection of supplying the public withgood receivers can be made.
The reasons publicly given by theB.B.C. in defence of their attitude-because defence it needs-is that ex-periments are proceeding on high -definition pictures, which would be somuch better, and therefore it would notbe worth while to go on with 30 -linetransmissions.
Quite frankly, I consider this attitudeof the B.B.C. to be technically inju-dicious and retrogressive as far as theinterests of television in this countryare concerned.
Everybody, even the uninitiated,would appreciate the fact that a Izo-line
Mr. G. W. Walton, the inventor of the Scophonysystem.
4
JANUARY, 1934
THE FIRST SCOPHONY PICTURESpicture is superior to a 3o -line picture.Speaking, however, with some measureof intimate knowledge of the problems
to the experts, the problem of syn-chronisation, which involves consider-able difficulties," and he further points
Vrophoor high -definition (120 lines) picture and sound transmitter with Mr. J. .Sieger of tie Scophonylaboratory.
involved in " putting over " such ahigh -definition picture, I would saythat it would be unjustified optimismto anticipate a regular high -definitiontransmission service, on the lines ofthe present 3o -line transmissions, inseveral months' time.
A GermanOpinion
The German experimenters haveshown at the recent (August, 1933)Berlin Radio Exhibition television pic-tures with a definition of 180 lines.One would have therefore expectedthat there at least one would find com-plete unanimity in favour of high -definition pictures. The more inter-esting it is, therefore, to quote anarticle published in " FerntechnischeUmschau " of " Der Radio-Haendler,"November, 1933, issue, in which aprominent Berlin television scientist,Dr. Skaupy, expresses his regret at thedecision of the B.B.C., which seems tohim a retardation of the developmentof television, and not progress.
Referring to the " 18o lines " Ger-man cathode-ray pictures, Dr. Skaupysays that while the number of picture -lines would be adequate, " there is,apart from the difficulty of transmissionwith ultra -short waves, and theirvarious disadvantages, also, as known
out " that the making of cathode-raytubes, which would preserve unvaried,over a long period, their small gascontents, which is essential for a sharpconcentration of a picture, is not at alleasy, and the observers of cathode-raypictures (as shown at the Berlin RadioExhibition) were, of course, not awareof the great and enormous troubleinvolved in their making."
Two Kinds ofTransmission
Interestingly enough, Dr. Skaupyarrived at a conclusion which, thoughalways held and on occasions expressedby the writer of the present article,should sound almost like a " heresy "when coming, as it does, from a writerliving in a country which at present,as far as publicly demonstratableresults go, has more to show in high -definition developments than thiscountry. He suggests that-pleasenote, even in Germany-there should betwo kinds of transmission, one of low -definition pictures, available for the largermasses and within their reach, owingto comparatively low price of apparatusand ease of handling, and one of high -definition, for the reception of which,for some time to come, more expensiveand more complicated apparatus, andaccordingly with an appeal to a dif-
ferent class of people, would be re-quired. He is of the opinion that thetwo services may exist simultaneously formany _years to come.
I completely endorse this view. Though,for instance, the laboratory of ScophonyLimited is engaged on the developmentof high -definition television, which,because it deviates from the normalroutine, may bring considerably nearerthe day when a comparatively simpleand reasonably priced high -definition(say, izo lines, the Berlin Radio Exhibi-tion having shown that even a 90 -linemechanical device was superior indefinition to a i8o-line cathode-raytube) receiver would be made availablefor the large masses, a 3o -line receiverwould undoubtedly for some time tocome be much cheaper than a i zo-linereceiver, and therefore more accessibleto the man in the street, and particu-larly to the technically -minded amateur.
Apart from this, there is the fact thatit may be years before the countrywould be covered with regional short-wave transmitters, radiating televisionprogrammes limited more or less to thevisual range. 30 -line transmissions can,
Small model of 3o -line television receiver, employingthe Scopbony rotating -echelon principle. Actualsize of television unit is easily visualised from thispicture. The picture shows also the size of themoving part used in this type of receiver, which isheld between the fingers of Mr. G. Wileleenhauser ofthe Scopbony laboratory. This model, used with amercury lamp, is capable of giving a satisfactoryprojected picture 34 in. by 81 in. and the movingpart can be driven and synchronised by the output of a
small valve.
however, be received over comparativelylong distances. (Excellent B.B.C. pic-tures were received on the Scophony
5
JANUARY, 1934
receiver in Manchester, and quiterecently, while in Vienna, I became therecipient of congratulatory appreciationof the quality of the B.B.C. 30 -linetransmission from the Chief Engineer
leading telephone firm, whoof a
Within the medium wave lengths' range;an increase of picture definition to, say,4o and perhaps even 6o lines may bequite feasible.
I would therefore submit for theserious consideration of the appropriatetechnical circles and the B.B.C. as apractical suggestion that for the nexttwo years at least the 3o -line transmis-sions should be continued, and thatthey should be treated as a serviceindependent of any progress made onhigh -definition pictures.
I would further suggest that thefacilities for receiving 30 -line trans-missions should not only not be cur-tailed, but more convenient receptionhours should be arranged. The B.B.C.may be well advised to consider theintroduction of television during theirnormal broadcasting hours as a pic-torial illustration and adjunct to ordinaryitems in the programmes. For ex-ample, the B.B.C. could with easetransmit on the television wave lengththe picture of the party leader debatingthe future of the British Empire, of
Chassis of large model of 3o -line receiver, employing prominent singers, lecturers, etc. Thethe Scophony rotating -echelon principle. addition of vision in such a way would
greatly enhance the value of the soundentertainment offered, and would prob-ably have a greater appeal than the" eleven - o'clock - at - night- half - hour -transmissions."
receives the London transmissionsregularly. (As a matter of fact, a 3o -lineB.B.C. picture on the Scophony re-ceiver is nearly as good as a 90 -linecathode-ray picture.)
FurtherImprovement Possible
Further, it is very likely that evennow the last word regarding low -definition pictures has not been said yet.They may be capable of further de-velopment on the present normal wave-lengths. The B.B.C. may perhaps bewell advised to try out on a somewhatshorter wave length than the one atpresent used, while still remaining
TheCost
There is one further aspect to beconsidered. Arguments have beenadvanced that the B.B.C. may not bejustified in spending several thousandpounds per annum out of the wirelesslicence fees on provision of televisionprogrammes for a small number ofpeople. Informal computations bringthe number of owners of various kindsof television receivers in this country
to between i 0,000 to 13,00o. I believe,therefore, that television is alreadypaying its own way. But even apartfrom this, very few subscribers amongstthe nearly six million owners oflicences would object to the B.B.C.spending in fostering television evenfive or ten times the amount it nowspends.
The development companies and thetechnical circles are justified in asking
Scopbony type of film, based on Mr. G. W'. thon'sstixograph invention. This invention is a novelmethod of pictorial representation, in which thepictorial limits of position and size of details areonly in one dimension. This type of film is of greatvalue in all cases, where there is motion of a picturerelative to apparatus. It is also of value withanimated pictures, as it is a true continuous record.Intermittency of exposure and movement of film iseliminated and the rate of movement of film is oneto five mm. per second, compared with the 450 mm.
per second of ordinary film.
the for an immediate recon-sideration of its attitude and a final,this time clear-cut statement. A posi-tive decision, giving a guaranteed con-tinuation of 3o -line pictures for a mini-mum of two years would enable inter-ested manufacturers to supply thepublic demand for efficient 3o -linesets.
Setting Outa Scanning Disc
AMATEURS who like to make asmuch of their apparatus as pos-sible will no doubt be interested
in the following method of making ascanning disc which was adopted byan enthusiastic reader.
The first procedure was to dividethe disc into thirty equal parts, and thiswas done by first making six divisionsby stepping the radius round the circle.Each of these divisions was thendivided into five by means of a pro-tractor. Next the disc was perma-nently mounted on the motor shaft by
HORIZONTALLINE
SUPPORT
This drawing shows bow the linesscribed on the disc.
SCRIBEDCIRCLES
EEDLE
IL
were accurately
means of the usual boss. A strip ofthin brass was then dug out of the junkbox and a line scribed down it about asixteenth of an inch from one edge, theline being then marked off to the picturewidth and divided into thirty sections.A centre punch mark was next madeat each section, and then the sixteenthof an inch filed off down to the punchmarks so that the result was a scale withslight indentations.
The sketch will explain the rest ofthe procedure. The motor was startedup, the scale rested on a block of woodand a gramophone needle mounted ina slip of wood held against the revolv-ing disc.
6
connect the neut.to effect vision reception.
In presenting the accompanying
enall
:an-nentibers
)w toeivers
1;21 P/ISICM
the Neonto Your Wireless Set
It picks up minute electrical impulses,which represent the vision -picturetransmissions, and when these areapplied to the neon -tube they add toand detract from the initial energywhich causes the neon to light. Thusthe brilliancy of theneon light is varied.
Knowing, now,
Fig. 1. (left) A cir-cuit suitable when thereis ample H.T. voltage.
pictorial sketches, to illustrate theseveral suitable systems of connection,no apology, therefore, is necessary.
Before attempting to interpret thedifferent circuit arrangements, it will,perhaps, be best to discuss the simpleprinciples involved.
First of all, the neon -tube is thatwhich supplies the light necessary forthe perception of the pictures by thehuman eye. This light source alone is
the modus operandiof the essential parts,we can go aheadand determine howbest to couple upthe apparatus toeffect the desiredresults.
We have alreadystated that there isan initial energy re -
SET
VAR. RESISTANCE
NEON TUBE.SWITCH ftJ
Fig. 2. An arrangement for use when the normal H.T. for the setis not sufficiently high.
not sufficient for vision -picture recep-tion, however, as some method ofmodulating the light-causing it toincrease and decrease in brilliancy-isnecessary.
The wireless set effects this operation.
quired to cause theneon to light up.The amount of en-ergy varies a little,but is usually in theneighbourhood of
170 to zoo volts at 3o milliamperes, andis known as the " striking " energy.This amount of energy is not usuallyavailable except in the case of mains -operated sets, or those making use oflarge super -power valves.
(Continued on foot of page 9.)
Pig. 3. Using a separate source of H.T. for the neon.
..114
POWER VALVE
Fig. 4. This is the arrangement when the wireless set has choice output.
7
TANLIARY, 1:934
Some Problem in
Cathodevy TelevisionIN the July issue of TELEVISION an article appeared on the application of
the cathode-ray oscillograph to television.
The developments of this new branch of the science are being closelyinvestigated, and this article is a forerunner of a series which will deal in apractical manner with the use of the cathode-ray tube and its associated circuits.
The production of the line screen atthe end of a cathode-ray tube is a com-paratively simple matter so far as theelectrical circuits are concerned, but the
lar source of trouble may arise from thefield magnet of a loudspeaker mountednear the tube. This may displace thebeam to such an extent that a consider-
VO COMP200V WV
62,1, ARAAAMAcr
2V
(A1,1024 AP
37, AAVAVAr.
technique of producing satisfactoryimages requires a knowledge of thebehaviour of the tube itself and of thevarious factors which affect the modu-lation and movement of the beam. Atypical double time base circuit is shownby Fig. I in which the movement ofthe beam in the vertical and horizontaldirections is actuated by a condenser-resistance combination dischargingthrough a thyratron.
n terferenceIt is important to remember that the
cathode ray is deflected by both electricand magnetic fields, and the majorityof defects in the formation of the screenitself can be traced to magnetic inter-ference from an outside source.
The earth's field is a permanent in-fluence in this respect, weak though itis, but its effect is offset by the fact thatit does not vary in magnitude or direc-tion, and hence it can be allowed forby orienting the tube or by applying aslight opposite bias from a weakmagnet placed nearby. Another shni-
be understood that the linear deflectionand response will be upset if the beamis given a " kink " at one point in itstravel.
MAGNET
C- BEAM
Fig. T. (left) Circuitdiagram slowing con-nections of double timebase to the cathode -say
tube.
Fig. 2. (above) Effectof strong magnetic field
on the beam.
able opposing deflecting force mayhave to be applied to bring the beaminto the centre of the screen.
The use of magnets for centring thebeam is not altogether advisable if astrong field is required, since they tendto introduce distortion in themselves,if mounted close to the tube. Thedrawing of Fig. 2 illustrates the effectof a concentrated magnetic field in thepath of travel of the beam, and it will
By far the most common source oftrouble is the mains transformer asso-ciated with A.C. receivers, or even theoutput transformer. The field sur-rounding these will give rise to an A.C.ripple, which will be superimposed onthe travel of the beam in either thevertical or the horizontal plane, pro-ducing a result shown by a typicalexample in Fig. 3. Here the inter-ference is affecting the vertical travelof the beam and appears as a wavy lineat the end of the screen formation.
Another form of interference pro-duces ripples in the lines themselves,and yet a third form produces light anddark bands across the screen, four in
number if the A.C. is 50 -cycle and the scanningspeed is izi.
This last effect can beturned to advantage insome cases since it enablesthe correct scanning speedof the horizontal time baseto be checked by astroboscopic method.
It might be wonderedthat the tube could not beprotected against externalfields by the use of a steelshield of high permeability,such as Mumetal. Thistt:,Fik 3. Effect of A.C. rippe on scanning screen.
8
JANUARY, 1934
is of course possible, but the thicknessof metal required adequately to shieldthe tube is such that the cost is almostprohibitive. There is also the risk thatthe shield itself may become acciden-tally magnetised and the remedy be-come as bad as the disease. On thewhole, the best procedure for mini -
Fig. 4. Illurtrating the thresholdeffect or slowing down of the beammovement at the centre of the screen.The method of obviating this is
described in the text.
mising interference from external fieldsis to mount the tube on a separatestand connected to the circuit by amulti -core cable. The most con-venient mounting is a long box, linedon the inside with thin sheet copper forelectrostatic shielding, the copper beingconnected to the anode of the tube. Afurther advantage of this arrangementis that the tube can be placed at a dis-tance from the receiver for focussingand modulation adjustments.
The best distance for viewing theimage at the end of the tube is about6-10 feet, depending on the size of theimage and the detail of the picture, andif the controls are placed at this dis-tance a better idea of the effect of thepicture on an audience can be obtained.
The remaining points are principallyconcerned with the scanning and syn-chronising circuits, but before considering these there is a peculiarity inthe screen produced by the cathode-raytube which will immediately be noticedby the experimenter at the outset. Thisis the presence of a bright cross in thecentre of the screen stretching for thefull length of the screen in both hori-zontal and vertical directions. Theeffect is somewhat as shown in Fig. 3and will clearly spoil the detail of thepicture at the centre of the screen.
" ThresholdEffect"
The cause of the cross is as follows :The majority of cathode-ray tubes at
present in use in this country aresoft " tubes-i.e., they contain a trace
of inert gas. The presence of this gasmaterially assists in focussing the beam,
owing to the production of positiveions by collision with the main electronstream on its way up the tube. Thesepositive ions drift into the main elec-tron stream and form a core whichtends to prevent the divergence ofelectrons from the main body, and thuskeeps the beam within the smallestpossible diameter.
Where the beam passes between thedeflector plates, however, the presenceof these positive ions is a disadvantagesince at very low deflecting potentialstheir charge will have to be neutralisedbefore the beam itself can be deflected.
This means that the response of thebeam will be definitely less to smallpotentials than those above, say, iovolts. On its to-and-fro travel, whenthe difference in potential between thedeflector plates of a pair is very slight,the movement of the beam will slowdown, and it will pause for a momenton " dead centre.' Theeffect will be the brighten-ing of the scanning line atthis point, and in the linescreen a series of brightpoints will appear as inFig. 4.
This effect has beenvariously named " Thres-hold Effect " or " OriginDistortion," and dependson the amount of gasin the tube as well ason the anode voltage andcathode current.
Since the trouble isan electrostatic one, it
obviated by shifting it to one side ofthe screen by a suitable bias applied toone of the deflector plates. The trans-verse bright line can be moved to thetop or bottom of the picture where itseffect is less marked, and the verticalline can be biased to the side.
The facility with which the screencan be moved by means of biasingpotential applied to the plates is of thegreatest possible help, and a simplecircuit, showing how the deflectorplates can have a small potential appliedto them relative to the anode, (Fig. 5.)
Part of the conventional time -basecircuit is shown, supplied from 150-300 v. H.T. Across the H.T. batteryis connected a high -resistance poten-tiometer, the slider of which is takento one of the pair of deflector plates(A2). A1, the " live " plate, is con-nected to the anode of the thyratron.Adjustment of the potential divider
Hi+o
H.T.-
DIODE
-OA
THYPATRON-0A2
I M
ANODE,OF TUBE/
Fig. 5. Bias potentiometer for deflector plates.
does not arise when magneticcoils are used for deflecting the beam,and this is one argument for their useinstead of deflector plates.
So far as the threshold effect is con-cerned in ordinary viewing, it can be
puts plate A, either at anode potentialor above it to the full value of batteryvoltage. The use of a similar poten-tiometer for the other pair of platesenables the beam to be moved to anypoint on the screen.
Connecting the Neon to YourWireless Set-Continued from page 7.
Fig. r shows a system of connectionsfor the neon when there is adequateenergy available, as in the case of tLetypes of receiver mentioned above. Itis the simplest arrangement possible.
Fig. a illustrates how an additionalH.T. battery may be adapted to increasethe normal H.T. energy available forthe working of the neon. Here, avariable resistance, preferably of about1,000 ohms, permits a very fine degreeof adjustment for the operation of theneon. This resistance -control arrange-ment, by the way, may also be used inconjunction with the two remainingsystems of connections.
Fig. 3 indicates a method of usingentirely separate batteries for striking
the neon-the r -t ratio output trans-former being employed to isolate thestriking energy from that which sup-plies the receiver.
Fig. 4, theoretically, is identical ineffect with Fig. 3, in that it affords a1-I coupling device between the setand the neon, as does a transformer.It is of advantage wherever a setalready includes a choke -filter outputfor the loudspeaker. All that is neces-sary is a further choke, similar in allrespects to that normally used forisolating the speaker from the H.T.supply to the receiver, and the appro-priate components common to theneon -tube supply.
In each case, it will be observed, theloudspeaker is omitted from the outputof the wireless set.
9
1"41..1`/141.01 JANUARY, 19 34
News from AbroadBy OUR SPECIAL CORRESPONDENTS
Italy - RomeNEW TRANSMISSIONS SHORTLY.
Ente Italiano par le AudizionoRadiofoniche informs us that at themoment transmissions of television on25.4 metres have been discontinued.New tests are expected to commenceshortly, but a definite date has not beengiven.
France - ParisTHE BARTHELEMY SYSTEM.
The well-known television researchworker, M. Barthelemy, recently gavea demonstration of his televisionsystem at the laboratories of the P.T.T.to M. Laurent-Eynac, the new Minister.M. Laurent-Eynac expressed his satis-faction as to the results achieved, andwe understand he issued instructionsfor regular television transmissions tobe commenced in the very near futurewith the co-operation of the adminis-tration of the P.T.T.
There is a general increase of interestin television in France.
United States of AmericaTWENTY-EIGHT TRANSMITTERS.
Great interest is being taken in tele-vision development in the U.S.A., andall leading laboratories are conductingintensive research to establish televisionas a commercial proposition. Licencesfor experimental broadcasts have beenrenewed, including N.B.C. transmis-sions from New York on 2,750-2,85okilocycles on a power of 5 kilowatts.Sparkes Withington, of Jackson, Michi-gan and the Western Television Re-search Corporation, had their licencesrenewed by the Federal Radio Com-mission.
At the moment there are twenty-eighttelevison transmitters in the UnitedStates, transmitting on the followingfrequency bands :---1,600-1,700 mega-cycles, 2,000-2,300 megacycles, 2,750-2,85o megacycles, 43-46 megacycles,6o -8o megacycles.
NEW AMERICAN TELEVISION SERVICE.
The Don Lee Television Broad-casting stations W6XS and W6XAO,Los Angeles, announce that from nowon there will bz transmissions of full-
length feature broadcasts of cinemafilms. This is now part of the regularschedule, and is in addition to theregular transmissions of news -reels andclose-ups.
To lessen sideband cutting which hasso far been experienced, the Don Leestation W6XS changed on Novemberfrom 2,15o kilocycles to z,800 kilo-cycles.
Dr. B. C. Goldmark, who was for-merly in charge of the television re-search department of Pye Radio, Cam-bridge, has been appointed head of theresearch division of the Ray -O -Tele-vision Manufacturing Corporation inLong Island City.
ULTRA -SHORT WAVES AND FOG.
In Boston, Mass., U.S.A. it has beenfound from experience that 56 -mega-cycle transmissions are greatly affectedby fog and mist. Whereas in normalweather no appreciable increase ofsignal strength has been observed withthe increase of power, in foggy weatherquite an appreciable increase has beenobserved.
STANDARDS AT WHICH To AIM.
Mr. W. Hoyt Peck discussed in arecent essay what constitutes perfectdetail in television. In his opinion,thc television image on 18o lines 18 in.deep will contain all details the humaneye can see when this picture is viewedfrom a distance of 9 ft.
Mr. Peck says : " Television maywell copy the use of an illusion of detail,of which the motion picture makes use.When a character is seen in a group ofa long shot, the features are more orless formless lights and shades, detailbeing supplied principally frommemory, and from very general im-pressions." Mr. Peck concludes :" Look for this the next time you goto the movies."
In this connection it is interesting tonote that the same opinion is expressedin a recent article published by theresearch engineer of the Leningradresearch department concerning tele-vision. It is said that it is useless toattempt to reproduce a perfect imageas the human eye will indicate perfec-tion in a picture, when actually it is farfrom being so.
Television Research in RussiaTelevision research is extensively
carried out in Russia by the LeningradElectrophysical Institute, and also theInstitute of Telemechanics. Experi-mental transmissions are being carriedout from transmitters in Leningrad andMoscow.
Germany18o -LINE TRANSMISSIONS.
It is reported that research workersin Germany are concentrating theirefforts on producing a commercialsystem of television, utilising 18o lines.It is stated that for transmissions on90 lines all difficulties surroundingtechnical requirements have been elimi-nated, but 18o lines have not yetemerged from the laboratory stage.The greatest difficulty at the momentis the modulation of ultra -short waveswith 8o -line television signal. It isexpected that when this difficulty isovercome, the first regular televisiontransmitter will be installed in Berlin.
It is stated that 18o lines constitutethe transmission limit on a wavelengthof 7 metres. In this connection it isinteresting to note that other researchworkers consider 18o lines to be themaximum number of lines required forsatisfactory television service.
It is urged that the authorities con-cerned concentrate on the type of pro-grammes the public wants, as, so far,this side of television has scarcely beentouched upon. Opinion in some quar-ters is that television programmes willbe nearly as important as the technicaldevelopment of the science.
HungaryA PROPOSED SERVICE.
The broadcasting authorities had anumber of television experiments con-ducted, and sent their television engi-neer to Berlin to gather the latestinformation as to the possibilities oftelevision broadcasting.
Regular transmissions do not yettake place, and no definite dates can begiven, but great interest is attached totelevision, and it is hoped to start trans-missions in the near future.
I0
3
JANUARY, 1934
Full constructional details of one of the most efficient receivers of the mechanical type which has so far been evolved.
Some Advantages of theMirror Screw
Compactness ; the image -formingscrew is identical with the size of theimage.
High optical efficiency providingbright pictures with small power.
Durable construction with littlelikelihood of later adjustment beingnecessary.
Very little strip effect.
IT is generally conceded that themirror -screw receiver is one of themost efficient that has ever been de-
signed. Moreover, it is one of thesimplest, with the further advantagethat it can be operated with quite smallpower ; in fact, the output from almostany medium -power three -valve set willbe adequate, the actual power requiredbeing from r to i4 watts. It thuscomes within the same class as the discreceiver so far as power requirementsand simplicity are concerned, but it isinfinitely better than the disc typebecause it is much more compact andthe general efficiency is greater.
Hitherto the mirror -screw receiverhas not been available to the amateuron account of several reasons, but nodifficulty will be experienced in theconstruction of the machine abour tobe described, and all the parts arereadily obtainable.
The photographs will show thatessentially the receiver consists of threeparts-the mirror screw, the motor andthe neon lamp. The mirror -screw iscomprised of a number of flat metal
plates with polished edges, and theseare to be built up into spiral formationand clamped on a special holder whichis then mounted on a steel shaft. Nodifficulty will be experienced in buildingthe screw, but detailed instructionsrespecting this will be given later.
It will be seen that the screw ismounted independently of the motorand is driven by a rubber belt. Thisbelt acts as a mechanical filter andprevents any small variations of motorspeed being transmitted to the screw ;it also permits of a compact layout andthe use of practically any motor.
Very little need be said about theactual construction, for apart from thecabinet this is merely a matter ofscrewing the motor and the bearingsfor the mirror spindle inposition. Ready built cabi-nets are available fromMessrs. Peto Scott, Ltd.,but if preferred this canbe made from the detailsgiven. All the necessarydimensions are given forthe positioning of the patts,and this will be furthersimplified if the large scaleblueprint which is availableat the price of is. be ob-tained.
The LightSource
As the mirror screw doesnot project, a line of lightis necessary which is viewedsubjectively (via the reflec-ting edge of the mirrorplates). This line of lightis easily obtained from theordinary flat -plate lampused with disc -type tele-vision receivers. and is easily
obtainable if the amateur does notalready possess one.
If this lamp is turned so that only theedge is viewed, a brilliant line of lightis seen. This line is masked off to.048 in. for the length of the glass bulbwith strips of lantern slide binding.The remainder is coated with blackpaint or covered with tinfoil (this helpsto increase the light efficiency).
The lamp is clipped in the holder andis placed temporarily close to the motor.It will be noticed that the light emerg-ing from the slit is of strip formation,but fairly wide. The lamp being easilyrotatable in the clips, the direction ofthe beam can be controlled.
It is, of course, directed to the mirroron the front panel of the chassis, and
This is a rear view of the miror-screw receiver with the outercase removed. The simple construction is apparent from this
photograph.
II
JANUARY, 1934
from there it is reflected to the edgesof the mirrors on the screw, and isrendered visible on each edge in turnas a spot. The lamp should be so
top or bottom. It will follow thatother ratios are possible by alterationof this distance, which is not verycritical. For instance, the German
The various parts of which the receiver consists are shown here ; note the reflecting mirror onback of the front panel.
the
N°2 BACLEARING
COUNTER-SUNK FROM
3' ou-rsi us.
Rf BASE34; THICK
Details of the chassis.
turned that the strip of light fills inline with the axis of the screw.
With the type of lamp and simplelayout used, the top and bottom of thepicture is not rendered at the samebrilliance as the centre. This can, ofcourse, be corrected by a concavecylindrical mirror instead of one of theplane type or a cylindrical lens can beinterposed between the lamp andmirror.
It is important to note that themirror screw should revolve in theopposite direction to that usual with amirror drum-i.e., the screw shouldtravel downwards when viewed.
In order to obtain a ratio of 7-3for the B.B.C. transmissions, the dis-tance of the lamp from the screw viathe mirror is adjusted so that the syn-chronising line appears either at the
The complete cabinet.
1-t
Side elevation of the cabinet.
transmissions on 1,600 metres are ofthe 30 -line 3-4 type viewed horizon-
tally. It is only necessary, therefore,to place the lamp nearer and 3-4 ratiois possible. It is also necessary toreverse the direction of rotation forthis transmission.
The parts for the screw are available,and the adjustment of the assembly isa fairly simple matter. It should benoted that adjustment in one planeonly is required, and that is the angularseparation. This adjustment gives theseparation of the spots correctly togive the horizontal scan.
To assemble and adjust the screwfirst obtain a board z ft. square andscrew a block of wood 3 in. square tothe centre. This has a vertical holeslightly under I in. into which ispressed the screw spindle so that it istightly gripped. From this centrescribe the largest circle possible on thebaseboard and divide the circumferenceinto thirty equal parts. This shouldbe carried out carefully.
Mount the lamp in a holder on thebaseboard so that the beam falls on thescrew at the axis and in line with thespindle.
9k.
The back of the cabinet.
Next arrange two slits on a battenas shown so that a spot is seen reflectedon a mirror edge from the lamp. Beforeadjusting, an arm is fixed to the mirrorscrew base ; the arm is free to revolveon the fixed spindle at one end, andthe other end is provided with an over-size hole for a woodscrew ; in this endis forced a needle to act as a pointer(see illustration). A woodscrew (withwasher) is inserted at A.
The needle is placed so as to coincidewith one of the radial divisions on thebaseboard and with the woodscrew andwasher at A the arm is fixed to thebaseboard. As the hole A is large,accurate setting of pointer can be made.All the plates should now be threadedon the carrier and the end plate made
12
JANUARY, 1934
finger tight so that the mirror platescan be just moved.
Starting with the bottom mirrorstrip, the spot from the lamp is pickedup in the slits. Fix in this position
PIECES OF PAPER
The lamp is covered with paper to provide thestrip of light.
SLIT SLIT
LAMP-/..
A check of the screw adjustment is made witha lamp.
with a little Durofix cement (obtain-able from multiple stores) at the endsand the base, taking care not to allowany to get on to the mirror edges. The
NEEDLE BRASS STRIP WITH HOLETO CLEAR SET SCREW ON
MIRROR SCREW BOSS
The indicating device for fitting the mirrors.
mirror strip can be held in contact bytwo small bulldog clips. Durofixdries rapidly, and in a few minutes thenext mirror can be positioned. Rotatethe screw by the arm clockwise so thatthe pointer is opposite the next radialline and screw to the baseboard. Thenext strip is moved so that the spot isagain picked up in the slits, and so on.
If desired, each mirror can be put onthe carrier and adjusted separately, andthis may be found easier. When com-pleted, the top end plate is screwed upand locked.
The cabinet has a hole at tie side,
The complete receiver is particularly compactand neat.
PARTS REQUIRED
Woodwork and lamp dip belt.Metal motor holder (Peto Scott).Motor type B.M.i (Mervyn).Parts used in screw (Mervyn).Spindle and ball bearings (Mervyn)Pulleys, resistance and mirror
(Mervyn).Neon lamp Osglim flat plate
(G.E.C., H. C. Sanders and PetoScott, Mervyn, etc.).
Suitable synchronising gear ifrequired (H. C. Sanders & Co.).
PAPER CLIPS
The mirrors are held temporarily in position withpaper clips.
and in this can be seen the ribs of theend plates. These are eight in number,and if viewed by light from 50 -cycleA.C. mains these will appear stationaryat 75o r.p.m.
The slot at the bottom is to enablethe lamp to be moved, and also theconnection to be made to it by a bayonetholder.
A variable resistance is shown on the
READTELEVISION
REGULARLY
front panel, and this is used for speedcontrol ; but a coarse speed adjustmentis obtained by another fixed resistancein series, the value of this dependingon the mains supply and motor used.
LINE OF LIGHT
The strip of light should fall in line withthe mirror axis.
BATTEN HERE
MIRRORSCREW
The mirrors of the screw can be set as shownabo.e.
A receiver of this type can be oper-ated under practically the same con-ditions as a disc receiver, which meansthat the power required is small. On
Showiag bow the picture ratio can be altered withthe mirror screw.
another page in this issue instructionsare given for connecting the neonlamp to the wireless set, and these willapply in the case of this receiver as wellas the disc type, so no difficulty shouldbe experienced in the operation.
In a later issue it is intended todescribe the fitting of synchronisinggear, but as this is accessory to thepresent apparatus it can be addedwithout alteration to the design ; alsoreaders may be assured that excellentresults are obtainable by hand control.
13
IELP.VISIONJANUARY, 5934
Patents and ProgressA
Synchronizing Apparatus(Patent No. 3991 54.)
To prevent a scanning disc or drumfrom " hunting " about its correctphase position, it is driven from themotor through an elastic link L ofindia rubber or similar material. Oneend is screwed to a flange A on thedriving shaft B, and the other end isscrewed to an extension of a flange F,which is concentric with the drivingshaft B and may be the scanning discitself. Provided the length of theelastic link L is large compared with
B
A mechanical coupling to prevent a scantlingdisc or drum "bunting."
the radius of the outer flange, thedriving torque increases very rapidlywith the displacement of the disc fromits correct position.-(C. L. Richardsand Baird Television Ltd.)
Scanning Drums(Patent No. 3995 5 2.)
In a scanning drum the larger thenumber of mirror elements, the better,though in practice a limit is set partlyby manufacturing difficulties, andpartly by the fact that the angulardisplacement of each mirror decreasesas the number of elements increases.
These difficulties are, to some extent,overcome by the arrangement shown,in which the mirror -elements M aremounted on an endless band whichpasses over two pulleys, A, B, of com-paratively small radius. Light fromthe picture P to be televised (or from a
Record o f Recent Developments
scanning aperture in reception) is pro-jected on to each mirror in turn as itpasses over the pulley B. Because ofthe small diameter of this pulley, thechange of angle of the mirror in its
A substitute for the mirror drum : the mirrorsare mounted on an endlesshand.
forward motion is correspondinglylarge. It therefore sweeps the reflectedray of light over a longer path on theviewing screen S, than is the case withthe ordinary type of mirror drumhaving a comparatively large diameter.-U. L. Baird and Baird Television Ltd.)
Assisting the Navigator(Patent No. 400279.)
The wireless beacon enables amariner to get his bearings at sea in
L1A Marconi invention to assist navigation.
foggy weather when it is not possibleto use the ordinary aids to navigation.There are various schemes for getting
this information across to the naviga-ting officer, some more simple thanothers, but they all demand a certainamount of care and skill in operation,particularly in judging the critical pointof maximum or minimum signal-strength, which gives the key to thedesired information. Also in mostcases it is necessary for the operator toknow the morse signalling code.
By means of television the ship'sbearings relative to the beaconstation,' can be indicated in the form ofa picture or image of a compass card.This conveys the desired informationdirectly to the navigation officerwhether or not he is a skilled wirelessoperator.
At the transmitting station a direc-tional aerial A, shown as a frame, ismounted on the same spindle as a scalemarked in degrees (or a compasscard). The aerial and attached scaleare rotated at a constant speed by amotor M. A lamp L is focused onthe scale just above the letters GB, andthe reflected light is passed througha scanning disc D, on to a photo -elec-tric cell C forming part of a televisiontransmitter T. The result is that ahighly concentrated beam of energysweeps round the horizon carrying withit an ever-changing compass readingand a constant representation of theidentification letters GB. Whereverthe ship may be located it will receiveonly that part of the scale which indi-cates the bearing of the ship relativelyto the beacon station. Synchronisingsignals for the television receiver aresimultaneously radiated from a separ-ate omnidirectional aerial so as tocontrol all receivers within range.-
P. Bowen and Marconi's WirelessTelegraph Co., Ltd.)
An "Iconoscope " Improvement(Patent No. 399654.)
In the Iconoscope type of trans-mitter, the ordinary single photo-electric cell, used for converting lightinto electricity, is replaced by a" mosaic " of cells. This consists of avery large number of silver globulesdeposited on a sheet of mica whichforms the anode of the cathode-raytube. The picture to be transmitted
(Continued on page ;o)The information and illustrations on this page are given with the permission of the Controller of H.M. Stationery Office.
14
JANUARY, 1934
The Eye in TelevisionBy J. H. Reyner, B.Sc., A.M.I.E.E.
TELEVISION as an entertain-ment is undoubtedly nearer thanit has been for some years. The
recent technical progress has broughthigh definition television within thebounds of practical politics, and theintroduction of really good qualitypictures will probably take most peopleby:surprise.
CILIARY MUSCLE
IRIS
LENS
how closely we can approach this idealin practice.
It is convenient to regard the eye asa form of camera. In the front of theeyeball is a lens made of special trans-parent tissue. It is oval in section, asshown in Fig. i, and the actual shapecan be altered by the ciliary muscle tosuit the different requirements. This
Persistenceof Vision
This phenomenon of persistence ismost important. Without it therecould be no television (nor, for thatmatter, any cinematography) becauseboth processes depend upon the pre-sentation of a series of pictures, one
FOVEA
OPTICNERVE
RETINA-'
Fig. 1. A diagram showing the various parts of the eye.
A
LENS
LIGHT SENSITIVECONES
6
RETINAFig. 2. Light is shown here entering the eye from two points
and producing two images on the retina
Let us consider what is involved inthe production of high grade pictures.In broadcasting we have realised for along time that the relative perfectionattainable depends very largely on thehuman ear, and research shows thatthis organ is a most accommodatingone. While a true vibration corre-sponding to a certain sound may bevery complex, we know that the earwill be satisfied with quite a poor imita-tion and will fill in much of the de-ficiencies by imagination.
The EyeAs a Camera
The eye is unfortunately much morecritical. It demands a much higherstandard of performance before it willbe satisfied. Moreover, we are alreadyaccustomed to reproductions of scenesin the form of photographs or cine-matography in which the standard isso high that the task of the televisionengineer is made still more difficult.Let us therefore analyse the mechanismof the eye and endeavour to estimatethe requirements for perfect detail and
operation is quite instinctive, andprobably most people are quite un-aware taat any such action is takingplace. The effect is that the focus ofthe lens is altered so that the rays oflight entering the eye from the scenein view are all accurately focussed onto the retina.
This is a sensitive surface at the backof the eye and takes the place of thephotographic film or plate in an ordi-nary camera. The mechanism bywhich the necessary intelligence is con-veyed to the brain is rather interesting.The whole of the retina is composedof millions of tiny cells which aresensitive to light. When any lightfalls on these cells a chemical changetakes place proportional to the intensityof the light, and this change is com-municated to the brain by the opticnerve. On the removal of the lightthe substance changes back to itsnormal state, but it is interesting tonote that it does not do so at once,there being a small delay in the actionso that the eye continues to see theobject for a small fraction of time afterthe light has been removed.
---after the other. In the cinema, in par-ticular, there is a period in between thepictures during which there is no lighton the screen at all, but if the rate atwhich the pictures follow one anotheris sufficiently fast the persistence ofvision entirely bridges the gap and theeye is not conscious of the change.
Now, one of the most importantfeatures of the eye in relation to tele-vision is its ability to distinguish detail,and we gain much useful informationby considering the exact mechanism ofthe eye *in Ithis respect. The mostsensitive portion of the eye as regardsdetail is that in the very centre of theretina where the small light-sensitivecells or cones, as they are called, areextremely closely packed. A littlethought will show that the closer thesecones are together the greater is theability to distinguish detail.
Consider two adjacent points, onelight and the other dark. Rays of lightwill enter the eye from these two points,as shown in Fig. z, and will focus them-selves on two spots on the retina nextto one another. Suppose that thedistance between these points exactly
15
JANUARY, 1934
corresponds to the distance betweentwo of the cones. One cone will/beaffected by the light and will communi-cate this intelligence to the brain. Thenext cone will not be affected becausepractically no light is falling on it, sinceit corresponds to the dark point. Thusthe brain can distinguish between thetwo points, and say that one is lightand the other dark.
A similar effect occurs if the distancebetween the two points on the retinais greater than the distance between thelight sensitive cones. In this case wehave a group of two or three conesin between the particular cones onwhich the light rays focus, and theintelligence is conveyed to the brainas before. If, however, the distancebetween the two points is less than thedistance between the cones, then theeye begins to fail. The particular conewill receive a small amount of light anda small amount of dark, and the onlyintelligence conveyed to the brain isthat there is some light coming fromthat particular area, but that is all.
Limit
the light rays focus on the centreportion of the retina. Around theedges of vision the acuity is much less,due to the fact that the cones are muchmore widely separated. Most peopledo not realise this in practice becauseif one wishes to concentrate attentionon any one object one naturally turnsthe head or the eye so that that par-ticular object comes in the centre of the
of DetailHence there is a quite clearly defined
limit to the closeness at which detailscan be distinguished. This limit isultimately determined by the distanceapart of the cones in the retina, andsince the retina is at a fixed distancefrom the eye lens, our limit is one ofan angle rather than an actual distance,this being known as the angle ofresolution.
This means that the farther away, themore apart two points have to be beforethey are distinguishable. If you maketwo lines in black ink on a card 1 in.apart, (Fig. 3) and you hold this card afoot from your eye, you can quiteeasily distinguish the two marks asseparate marks. If you place the cardzo to 25 feet away, you will find thatyou cannot distinguish the two marks,but you can only see one blackishblurr. It does not even look as blackas it did when it was close to the eyefor the reason already given, that lightrays from the black marks and the sur-rounding white portions are all fallingtogether on one particular light-sensi-tive cone, which therefore only registersthe average value of the light received.
VisualAcuity
Visual acuity (ability to distinguishdetail) is more marked for objects inthe centre of one's field of view where
Fig. 3.the two
1 IIf this diagram be viewed from a distancelines will be indistinguishable as being
separate.
field of view ; but if you think it outyou will realise that you cannot seedetails anything like as well at the sideof your field of vision.
In the centre of the retina-theregion known as the fovea-the conesare about .003 mm. apart, and since the
Fig, 4. The "grain" of 120 -line scanning. Ifthis picture be held about 3 ft. away the detail willappear perfect and at this distance the eye could not
distinguish more if it were present.
Readers who have experimented withphotography will know that with alarge aperture lens a soft focus effectis obtained, really sharp focussing onlybeing possible with a small aperture.
In the same way the eye, underconditions of poor light, cannot dis-tinguish so much detail, and experi-ments made by the Kodak Co. underthe conditions obtaining in an averagecinema indicate the angle of resolutionas being about twice the daylight value-i.e., about 1-5oth to i-6oth of adegree. With television the visualacuity is probably still less, but thefigure quoted will serve as a convenientstandard.
Let us consider a picture z in. highand 21 in. wide, at a distance of zo in.The closest distance which can bedistinguished under such conditions isabout .0057 in. If we assume hori-zontal scanning, this requires z .0057=350 lines. This is still some way inadvance of modern technique, but itmust be remembered that this repre-sents perfect detail. We know thatalthough the eye is relatively un-accommodating, it will accept coarserdetail than this.
focal length of the eye is about zz mm.this corresponds to an angle of aboutz8 seconds of arc-i.e., about Thdegree. This represents the angle ofresolution for objects in the centre ofthe field of view under normal lightingconditions. But in television we aredealing with a much reduced illumina-tion, and the effect of this is to reducethe visual acuity, because the irisexpands to admit more light.
The iris is a circular diaphragm overthe lens which automatically adjustsitself to admit the optimum amount oflight. In strong light the iris contractsto leave a very small aperture for thelight to enter the eye. In dim lightthe iris expands to its full aperture of
in. or more to admit all the lightpossible, but in doing so the focussingpower of the eye lens is reduced.
The Circleof Confusion
The home cinema affords proof ofthis fact. The size of a typical 16 mm.frame is ro.5 by 7.6 mm. We knowthat the various points do not focussharply, but have a finite size which isknown as the circle of confusion, and isusually taken as about .o5 mm. Thiscorresponds to 15o lines, which isquite feasible. In fact, experimentalcathode-ray television has been carriedout on 18o and zoo lines in manyquarters. Hence modern television isapproaching a state with which the eyewill be well pleased if not completelysatisfied.
One final point is worth remember-ing. Since the visual acuity of the eyeis determined in terms of an angle, itfollows that the farther away one isfrom the picture the better is the detail,or, alternatively, the less the number oflines necessary to provide a givendegree of definition. As an example,it is easy to show, from the figuresalready given, that a 3o -line image31 in. high and 1 in. wide viewed ata distance of 14 ft. gives perfect detail.In other words, even if the detail weremade better, the eye could not appre-ciate any improvement at this distance.
The moral, therefore, is do not gettoo close to your pictures particularlywith low -definition reproduction.
JANUARY, 1934 'filitY1S410fl
REVIEWS OF THE PROGRAMMES AND RECEPTION REPORTS
AS technique improves, artistspresent a more human appear-ance in the studio. Make-up
has been modified now that lightingis more precise, and a soubrette nolonger looks grotesque as she standsin the dazzling shaft of light. Hereyebrows are not so thickly blackenedand her lips are not so full of paint.
In the earlier days when pictureswere cruder, I failed to recognise afriend through her coating of grease ;that could not happen to -day.
Current fashion prescribes a noselightly shaded in pale blue on eitherside, eyebrows thinly emphasised inblack, eyelashes beaded with mascaroand lips touched with blue. Formid-able as this sounds, the effect is notunpleasant in the twilight of the studio,and is not unnatural even when lit bythe beam from the projector.
Twelve months ago the same artistwould have appeared with the pallorof a ghost, the black eyes of a pugilistand the mouth of a coon. The refine-ment is made possible by additionalphoto -electric cells and experiencewhich has taught the engineers justwhere to place them to secure the bestresults.
* * *Those who receive the television
programmes are familiar with thestairs used for revue and other spec-tacles. Twelve feet long, the staircaseis usually placed against the backscreenfacing the projector. For the panto-mime it is being shown in a new posi-tion. Placed across the studio withcanvas pinned to the side which isnormally invisible, we see Cinderellatripping up to quit the ball and leavingher slipper behind. The staircase is auseful prop, but I had begun to tire ofit; then the producer added castorsand used it as a trolley. Here is afresh variation.
* * *
Once or twice recently the picturehas not been up to standard, and when
the pipers of the Scots Guards were inthe studio their image disappearedaltogether for a few seconds. Thecause is still uncertain. Lookers -inmay have blamed their sets.
A report referring to the programme onDecember 18 which has been received saysConsidering fading was noticeable, thepicture at times almost disappearing,whilst at other times the contrast of lightand shade was very strong.
Although she has taught manySpanish dancers who have been seenin the studio, Elsa Brunelleschi wasmaking her first appearance. In hergypsy dance the picture seemed ratherdim at first, but this was intentional,and the light improved as she mounted
INTHE
STUDIO
ATBROAD-
CASTING
HOUSE
THE
ARTIST
IS
MISS
JANECARR
the stairs and caught the rays of the" sun."
Technically, the outstanding featurein " Cinderella " on December 27 wasa slick fade from a property pumpkinin the studio to a coach and horsesdrawn in black and white on a card inthe miniature transmitter. The fairygodmother taps the pumpkin with herwand and the vehicle arrives at thetrot. Like the cinema, television lendsitself to an optical trick.
* * *
The small transmitter is now beingused for larger drawings which areslowly pushed through the frame as aslide is pushed through the light of amagic lantern. In this way the draw-ing is scanned in sections, a develop -
17
JANUARY, 1934
ment which gives greater scope to thedraughtsman.
In contrast to the process in the mainstudio, the projector of the small in-strument is fixed, and it is the objectbeing televised which has to move.
Alanova was at Broadcasting Housewatching the picture while Agnes deMille was dancing. It was a strikingperformance, and afterwards Alanovaremarked that better perspective wasobtained for dancing in television long -shots than was possible on the screen,which confirms Adeline Genee'sopinion, given after her world farewellin the studio, that the film is not asuitable medium for ballet work.
* * *
Some ingenious arrangements oftoys were seen in the seasonal toylandprogrammes during Christmas week.Yvette as a doll and E. Kelland-Espinosa as a golliwog mingled withTeddy bears, penguins and trains froma London store. The golliwog cameto life when Father Christmas hadwound him up, and we discovered thatthe big doll could sing.
* * *
The year ends with the news thatthe B.B.C. is to continue these 30 -line
Shabalevsky and Nina Baronota in Jeux d'Enfants.
programmes after the end of March,probably on two nights each week.So there is no reason for lookers -in tofear that they will have to scrap their
experiments at Br oadcastingHouse.
Olive Groves, *popular radio
The photograph shows a scene in a Broadcasting House studio during the production of atelevision revue. Harry Pepper and John Watt directing the production.
apparatus. Although research is con-centrated on ultra -short-wave systems,
I believe that 30 -line visors in useto -day will beneeded for a longtime to come.Meanwhile plansare being dis-cussed for a newtelevision studioto replace B.B.in the basementat BroadcastingHouse, whichwas originally de-signed for use bya dance band.This develop-ment is impor-tant, as it impliesthat the B.B.C.has not lost faithin studio work,though film isbeing used forresearch.
Early in the NewYear the E.M.I.system of filmtransmission willbe installed forultra -short - wave
soprano ; Anna Duse, in her In-fanta dance again; and Kassen, theRussian singer, were booked for Dec-ember 29. A new dance act by Ray andGeoffrey Espinosa is down for NewYear's Day, when Anona Winn andDimitri Vetter will also be seen andheard.
Harold Steam, a lieder singer wasrecommended by Delysia, besidesHermione Daneborough, classicaldancer from the Vic -Wells Company,and Aranka von Major, who sings inthe " Cafe Colette."
Jack Browning has found a newdancing partner in Marjorie Stevens,and they are down for performance onJanuary 3 with Louise Maxim, con-juring and juggling, and Betty Bolton,our old friend.
Comedy on Januarys will be en-trusted to Arthur Askey. His songsand patter were amusing last time.
A Letchworth reader writes : Lastnight (Dec. ii) reception was better than ithas been for a long time. I was able to seeevery item except the first one. This wasmy fault as I forgot to allow for the motorto warm up. I general?), use cathode-rayequipment although I can obtain quite goodpictures on a zo in. Mervyn disc and a smallOsglim indicator neon. Brookmans Parkis about twenty-five miles from here, so I donot experience any trouble from fading.
18
JANUARY, 1934 IELPNISION
A Novel Receiving SystemBy E. L. Gardiner, B.Sc., F.T.S.
Here are the first published details of a novel system of reception which is now undergoing experimental tests.
IT is interesting that the Nipkowscanning disc, probably the veryearliest device used for successful
television, remains to this day the mostwidely used scanning device forreceivers. This can be attributed toits several very good features, promin-ent amongst which are the highaccuracy of the scanning obtainable,the low cost, simplicity and relativeease of synchronisation of a good disc.In fact the disc has only one seriousdrawback, unfortunately a very vital
one, which is the poorness of theillumination which it gives when usedfor systems employing any considerable
number of scanning lines. Whereasin the case of 3o -line television a discimage may be bright enough for gooddirect viewing, it cannot be madebright enough for satisfactory projec-tion on to a large screen even when thebest light sources so far known areemployed behind it.
This disability of poor illuminationbecomes rapidly more serious when alarger number of scanning lines areemployed, until at I zo lines for examplethe image is hardly bright enough for
This is the firstexperimentalmodel of thenovel receiver ;note the minia-ture neon in-serted in the
disc.
really satisfactory direct viewing. Therapid falling off in brightness is partlydue to the increasing smallness of the
holes, but it is also bound up with thequestion of the very inefficient utilisa-tion of the available light source, whichhas to be diffused over the whole of theimage area on the disc, or made largein area as in the case of a plate -typeneon tube.
Only a minute portion of the totallight is being employed to form theimage at any instant ; in the case of3o -line television this fraction is of theorder of one -twentieth per cent. Fori zo-line working, however, it becomesas low as one three -hundredth per cent.or less, causing a terribly low efficiency.
IncreasingLight Efficiency
It is largely because of this deficiencyof the original disc that other scanningdevices, such as the mirror drum, havecome into increasing use. Most ofthese methods are characterised by thefact that nearly the whole light fromthe light source used is being employedduring the whole period of scanning,resulting in a greatly increased brilli-ance and permitting of screen projec-tion. They are also better suited tothe use of light sources of high intrinsicbrilliance, such as the crater tube orKerr cell.
An interesting system invented bythe writer is at present undergoingdevelopment in the Wilson Labora-tories, which is able to do away verylargely with this main defect in the disc
NEON TUBES
EXCITER COIL MOTOR
DISC
EXCITERCOIL
NEON TUBE
'PROJECTING LENS
DISC OF NON -CONDUCTING SCREEN
OSCILLATOR
MODULATEDVALVE
MATERIALThese two drawings show the schematic arrangement of the new receiver in elevation and plan. In the plan view the optical system is3indicatea.
19
JANUARY, 1934
as a television receiver, whilst retainingits good features ; it also makes itsuitable for screen projection on amore competitive basis with the newersystems.
The essence of the system is the useof separate light sources in place ofeach hole of the usual disc ; a very oldidea in itself, but one which has notbeen applied with any success in thepast owing to the crudeness of themethods used.
Discs have been made at varioustimes in which ordinary electric lampswere used in place of the conventionalholes, operated in their correct orderby means of some form of commu-tator which rotated with the disc andconnected the right lamp into circuitwith the television signal currents atthe required moment. Whilst fila-ment lamps are obviously of little usefor this purpose, owing to their slug-gish response to changes of current,some results might be obtained frommodern gas -discharge lamps were itnot for the many troubles introducedby the commutator, a device which isalmost incapable of giving perfect andconsistent connection at high speed.necessary in television. There areother defects in such a system for alarge number of lines which aresufficiently obvious to show its im-practicability as a commercial receiver ;
there are, for example, the cost andcomplexity of the commutating mech-anism, the cost of the many lamps, andthe great difficulty of providing anumber of lamps containing electrodeswhich have identical electrical charac-teristics, and which will remain ofsufficiently identical brilliance after longuse.
No ElectricalConnections
The system about to be described,however, overcomes these difficultiescompletely. In the first place thelamps used at the present stage ofdevelopment contain no electrodes,and have no external connectionswhatever. Also no mechanical com-mutating device is used. The lampssimply consist of very small glass tubescontaining neon or a suitable gasmixture at low pressure, and they arecemented through the holes of the discwith their long axes parallel to thespindle of the disc. One end of eachtube is blown of clear glass, and carriesa suitable square or hexagonal dia-phragm. Such tubes are small, light,and cheap, and if filled with gas at thesame time, have practically identicalcharacteristics.
The lamps are caused to glow bythe action of a high -frequency field,provided by a valve oscillator modula-ted with the television signal. Theirbrightness is thus a function of thesignal at any instant. This is effectedby the periphery of the disc on which
Here is a close-up showing the coil in whichthe high -frequency field is produced.
the image is to be formed and on whichthe tubes are placed passing between theturns of an inductance carrying theradio -frequency current ; thus thetubes are excited without any metallic
contact with the disc, and thereforewithout friction or irregular contact.
A very important feature of thearrangement is that only the tube whichis contributing to the image at anyinstant is within the field and glowing,all others (except, possibly, those adja-cent to it) being extinguished, andmoving rapidly through the cold air.Hence in the case of 3o -line working,each tube has a period nearly thirtytimes that for which it is alight inwhich to cool off before it is againcalled upon to glow.
The absence of electrodes within thetubes make them capable of with-standing high temperature withoutsputtering or damage, and hence highbrilliance can be obtained which isstill further increased by the fact thatowing to the efficient cooling the tubescan be run while in motion at a brilli-ance which would destroy them ifapplied for more than a few seconds toa single stationary tube. These factorsmake for good illumination, and whileit is not yet known to what extent thebrightness of the tubes can be increasedby suitable design and the use of a verystrong field, preliminary work indicatesthat projection on to quite large screenswill be possible.
Those readers who are interested inthis development of disc working willhave an opportunity of seeing the re-search model of the system at theexhibition of the Physical and OpticalSociety, which opens shortly after thepublication of this journal. It ishoped to describe future progress ina month or two.
The scanning disc used in the German daylight transmitting system.
20
JANUARY, 1934
THE TELEVISION ENGINEER
THE THEORY OF THE KERR CELLThis isIn this,
the third of a series of articles on the action of the Kerr Cell.the author, J. C. Wilson, discusses the interaction of electricity and light.
WE now come to the considera-tion of the interaction of elec-tric phenomena and light.
Nearly a hundred years ago, whenFaraday was pursuing his researchesinto electromagnetic action, he chosefor examination a feature of a ray oflight which is extremely sensitive to
effect ; it may be observed in mediaand under conditions somewhat dif-ferent from those with which Faradayhimself first saw it. For example, if aray of plane -polarised light, proceedingfrom a Nicol or other similar device,be passed through a cylindrical vessel,surrounded by a coil of wire, and con -
a C
Fig. I. A diagram showing the effect of polarisation.
ANALYSER
observation : namely, its state of polar-isation. His apparatus was, by modernstandards, exceedingly crude and, aftermany experiments, in his diary he hadto write " no effect." (It is instructiveto notice the strength of his convictionthat an effect of the kind he was lookingfor existed : a favourite expression ofhis after an experiment which, withmore delicate apparatus, would haveyielded a result is : " Think it ought tohave done.")
On September 13, 1845, his persist-ence was rewarded : on passing a rayof plane -polarised light through a blockof heavy glass (lead borosilicate) situ-ated between opposite poles of anelectro-magnet, so that magnetic linesof force traversed the glass in the samedirection as the light, he found that theplane of polarisation of the light wasdeviated slightly when current was sentthrough the electro-magnet coils. Hiscomment is pithy : " This fact willmost likely prove exceedingly fertile."
Rotation of thePlane of Polarisation
This deviation or rotation of the planeof polarisation of light in media undermagnetic stress is termed Faraday's
taining carbon disulphide (which is aclear, yellowish, evil -smelling liquid atordinary temperatures) so that the lightpasses down the axis of cylinder andcoil, a second Nicol can be set to absorbor " extinguish " all the emergent light.
This setting will, however, not becorrect for total extinction when acurrent is passed through the coil, andthe second Nicol will have to be twistedslightly, by an amount varying with thestrength of the current, completely toabsorb all the emergent light. If, on
is passed. This system was proposedas a light -modulating device for tele-vision in 1884 by Paul Nipkow, whosename is associated with the scanningdisc.
For convenience in reference, theinitial Nicol or other polarising deviceis termed the polariser and the secondthe analyser in an optical system of thekind just described.
The KerrEffect
About thirty years later an effect ofa totally different character, arisingfrom the action of electric force onmedia through which light is passed,was discovered by Professor Kerr. Anarrangement for observing this effectmight consist of an analyser and polar-ise; between which is placed a glassvessel containing a medium such ascarbon disulphide. In the liquid areimmersed two parallel metal plates ashort distance apart, so that the lightfrom the polariser can pass betweenthem.
If the analyser is set to extinguish allthe light coming from the polariser,and then an electric potential differenceis created between the plates, light willpass through the analyser again, pro-vided that the plates are correctlyoriented with respect to the plane inwhich the light issuing from the polar-iser is polarised. This effect, knownas the Kerr electrostatic effect or Kerreffect, differs radically from the Faradayeffect in that no new position of theanalyser can be found in which the light
Fig. 2. Slightly increased sensitivity may be obtained by the arrangement shown here.
the other hand, the second Nicol is set,with no current in the coil, to pass thelight from the first and left in thatposition, then it will no longer pass itat all, and the strength of the trans-mitted light will vary, when a current
is again completely extinguished, andwe must now proceed to investigatethe nature of this difference.
When a liquid such as carbon di-sulphide or nitro -benzene is subjectedto electric stress (that is, when elec-
21
trodes immersed in the liquid are con-nected to opposite poles of a source ofelectro-motive force), the molecules ofwhich it is composed behave as thoughthey formed part of a crystalline sub-stance like Iceland spar : that is to say,the liquid assumes some of the pro-perties of a doubly -refracting or bi-refringent medium. A ray of lightentering such a stressed liquid splitsinto two components, one of whichtravels faster through the liquid thanthe other, although since both com-ponents travel in the same path, theycannot be seen as separate beams.
vibrations are slightly out of phase,yielding, instead ofplane-polarised light,elliptically polarised light.
The analyser can pass only lightpolarised in one plane ; in the case ofelliptically polarised light becomingincident upon it, therefore, there is noposition into which it can be turned sothat no light gets through it (as wouldbe the case with plane -polarised lightincident on it), but there is a positionin which least light gets through-i.e.,when the plane of polarisation passedby the analyser coincides with theminor axis of the ellipse representing
50%
rna 40rn
rn 30
E: 20
-I 10
0 500 ImoVOLTS
Fig. 3. The theoretical characteristics of the Kerr cell.
1500
Each component is polarised, and theplanes of polarisation of the two com-ponents are at right angles.
Now when a ray of plane -polarisedlight enters a stressed liquid along adirection at right angles to the stress,the plane of polarisation being at 43°to the direction of the electric lines offorce, the component vibration (a term ofwhich' the meaning was explained inour first article) polarised in a planecontaining the direction of the line offorce separates out : it is yi or 0.7071of the intensity of the original beam.
Another component vibration, polar-ised in a plane at right angles to that ofthe first component, and of the samemagnitude as the other, also separates,and no ray polarised in the originalsense is left to pass through the liquid.If these two components travelledthrough the liquid with equal velocities,then they would combine again, atemergence, to form a beam of theoriginal intensity, polarised in theoriginal plane, so that no net effectwould be observed. However, theydo not travel with exactly the samevelocity, but one loses a little on theother so that on emergence their
the elliptically polarised light comingfrom the liquid.
The diagram, Fig. 1, will help tomake this clear.
A very simple optical system, con-sisting of a polarising device, polarisinglight in the plane indicated by the arrowon its end -face, a lens, a glass vessel
JANUARY, 1934
ceeding from the polariser). Fromwhat has been said in the precedingarticles, it will be clear that normallyno light will be passed by the analyser,for all the light reaching it is polarisedin precisely the wrong plane.
Suppose (a) is a diagrammatic repre-sentation of the state of polarisation ofthe light coming out of the cell whenno potential difference exists betweenthe plates. The state of polarisationwill change to elliptical polarisation asshown at (b) when electrostatic lines offorce traverse the medium between theplates ; as the potential differencebetween them increases, more andmore lines are added, until a point isreached, represented at (a) when thelight is circularly polarised, and in thisstate the analyser could be turnedaround the axis of the beam withoutchanging the intensity of the lightemerging from it.
Further increase in potential pro-duces elliptical polarisation again withthe major axis of the ellipse canted ina different direction as shown at (d),and finally a maximum is reached whenall the light is passed by the analyser,since the ellipse contracts again into astraight line representing plane polar-isation, but in a plane at right angles tothat in which it was originally polarisedon leaving the polariser.
How important it is in designing atelevision receiver to understand thereal nature of the Kerr effect will beobvious from the distinction which hasnow been drawn between this and theFaraday effect or rotation -of -plane -of-polarisation. Yet again and again wehave read quite eminent authorities'descriptions of the Kerr effect in whichit has been wrongly stated to dependupon rotation.
Substance. KerrConstant. Colour.
SpecificResistance,ohmlcm3.
DielectricConstant.
Nitro-benzene-Commercial ... z2 x ro-6 Yellow 5 X 107 36.4Best Purified ... 41 X io-6 Slightly
yellowishi x ro10 38.4
Meta Nitro -toluene 1.43 X 10-4 - - 29.3Carbon Bisulphide - Nearly
colourless- z .6 z
containing, for example, nitro -benzene,immersed in which are two parallelopposing plates, and an analysingdevice permitting only that light topass which is polarised in the planeindicated by the arrow on its end -face(that is, light polarised in a plane atright angles to that of the light pro -
Kerr Effectin Liquids
We must now turn to an investiga-tion of the conditions under which theKerr effect is best developed in liquidmedia : first of all, we give a list of the
(Continued on page 37.)
22
JANUARY, 1934 11-1.2VISIOn
THE TELEVISION ENGINEER
Picture Shapes and Scanning LinesBy Robert Desmond.
Standardisation of picture ratio is a matter needing early settlement.This article explains the various considerations upon which a decision depends.
In the reproduction of any scene theresulting picture has always to be fittedinto some mask or shape. Nearlyalways a rectangular shape is chosen.Very few circular and oval masks areused, excepting perhaps by the profes-
2
3
4
5
6
7
C
DIRECTION OFSPOT TRAVERSE --
Fig. I. This diagram shows bow detailmay be lost.
sional photographer of some twentyyears ago. This fact is rather sur-prising, as if you question any of yourfriends as to what shape of scene theireyes seem to view, they will generallyagree that it is an oval in a horizontaldirection, which is to be expected whenone considers the " layout " of thehuman eyes. In spite of what oureyes may or may not frame a scene by,the rectangular mask is predominant.
In mechanical reproductions certainstandard shapes or sizes have beenadopted for commercial reasons. Ofall the shapes that are available, how-ever, four have withstood the test oftime to a far greater extent than anyothers, the ratio of their sides being
: 1.3 to 1.4.Having decided on a picture shape
one then has to decide whether it isbetter to have the greater length in thevertical or horizontal direction. Innearly all reproductions .the mask canbe turned to suit the subjeci but where the greatest number of iccnes are
reproduced as in cinematography theratio I : 1.165 is fixed to give a breadthgreater than the height, the actual sizebeing 2.1 X 1.8 centimetres on the film.(These figures are for the " talkie "picture of today.) In the days of thesilent film the mask was 4 x 3 whichwas very pleasing for outdoor subjectsbut a single " close-up " of a face orsingle figure of a person had a con-siderable amount of wasted picturearea on either side and great care hadto be taken not to include anything thatwould detract the eye from the mainobject. In the early days of motionpictures many sizes of films were usedbut it was quickly realized that stan-dardisation had to take place and viewedgenerally the choice seems to have beenwell made.
ManyComplications
History seems to be going to repeatitself with television, but in addition tovarious picture ratios there is the
made. But what standards will beadopted ? From past experience onecomes to the conclusion that somethingof the order of the r : 1.3 picture ratioseems to have the strongest claim withthe special advantage of linking tele-vision with the film.
While the adoption of a picture shapeis relatively simple, the number of scan-ning lines is a most difficult problem,.as on the number of lines depends theamount of detail reproduced.
Theoretically it can be shown thatany television picture requires frequen-cies from zero to infinity for perfectreproduction. Luckily, in practice onecan work with a frequency band ofwhich the lowest frequency is that ofpicture speed and the highest about
the first zero fre-quency. The term " zero frequencyperhaps is new to readers and should beexplained. It is obtained by multiply-ing the number of times the linearlength of a square spot divides into thebreadth and height of the area scannedby the picture speed. For example in
Fig. 2. These two figures show a transmitted object and tie received reproductionrespectively. Portions of the latter are distorted due to the process of scanning.
further complication of the number ofscanning lines and the direction inwhich they scan the picture area. Asin the film industry, however, stan-dardisation must come with televisionbefore any real commercial advance is
the case of the thirty line picture radia-ted by the B.B.C. one has 3o (breadth)X 7o (height) x 12.5 (picture speed) =26,25o cycles.
Physically the first zero frequencynever exists as a fundamental. In,
23
JANUARY, 1934
Fig. 1 we have a series of black andwhite lines of equal depth separated bywhite spaces of a depth equal to thatof the black lines. Now if these alter-nate black and white areas are scannedby a spot whose depth is equal to oneblack and one white line together, itwill be clearly seen that after the side ofthe spot CD has passed the lower edgeof the sixth black line no change willtake place in the value of the light inthe spot till the side CD has left thebottom edge of the first black line (thespot is travelling upwards), and thepattern of the six lines will appear onthe receiving screen as a mass whosetone value will be in the middle of thescale of tone values.
From the above it must not bethought that it is impossible to transmitdetail such as the black line No. 7which is the same depth as the linesabove it. If this line was the breadthof the picture it would produce afundamental frequency of 375 in theBaird system but ifwe had 7o such linesequally spaced across the picture thefundamental frequency would be 26,250the zero frequency.
Scanning Linesand Definition
It is generally assumed that anincrease of scanning lines give betterdefinition, but this is not always thecase as will be shown. If the numberof lines were doubled in the presentpicture ratio the first zero frequencywould be 140 x Go X 12.5 = 105,000c.p.s., but if on the other hand ..hepicture was square, the zero frequencybecomes 45,000, while if the picture hada i : z ratio the spot travelling acrossthe shortest dimension gives us6o x 3o X 12.5 12,500 cycles which isless than half the present 30 -linepicture at present being broadcast.From the above it will be obvious thatwhen deciding on the number of linesto be used in a picture the amount ofdetail they will transmit will dependfirst on the picture ratio and secondlyon what direction the picture, if otherthan square shaped, is scanned.
In this country the television picturewhich is broadcast is scanned vertically,while without exception, to the writer'sknowledge, horizontal scanning is usedelsewhere. It is not perhaps renli7edthat there is always more detail repro-duced in the direction of spot traversethan that of strip traverse. This isobvious when one realizes that in thedirection of spot traverse the spot mayoccupy an all but infinite number ofpositions, while that of the line may
only be a definite amount. Further-more the spot traverse can be consider-ably improved by certain electricalnetworks which the television engineerterms aperture correction.
On the left-hand side of Fig. a wehave a pattern before being televisedwhile on the right is the same patternas reproduced in a receiver. It will benoticed how the pattern which runsparallel with the scanning lines and doesnot fit them exactly is distorted, whilethose that do fit are reproduced cor-rectly. That part of the design whichis at right angles to the scanning lineis reproduced correctly by virtue of thesliding spot with the addition ofcorrecting networks and the diagonalpart reproduced with a form of dis-tortion which causes a saw -like appear-ance. Incidentally readers who getthis saw -like edge to similar lines intheir visors may be interested to knowthat their receiving apparatus is passingthe higher frequency components ofthe signal.
Many people often express adversecriticism on the shape of Mr. Baird'sfirst picture. The choice, however, is
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ensure the delivery of "Tele-
vison" regularly each month.
well made. 'When the first practicalhalf tone television was achieved onlya face and a little of the neck wereattempted and the 3 X 7 picture ratiofitted such a subject admirably. Asthe essential detail of a face lies in thehorizontal direction it is best scanned atright -angles, hence vertical scanning,while the limit of thirty lines was madeby the sensitivity of the photo -electriccell and the desire to keep the frequencyband within reasonable limits to com-mence with.
StandardisationEssential
Standardisation of the picture in thefuture must come to ensure the com-mercial success of television as it had toin the film industry. Undoubtedly theshape will follow that of the cinema,especially as all the high -density tele-vision pictures of today are reproducedfrom film pictures ; nor must it beoverlooked that film interests controlquite a few television laboratories.
As to the number of lines, anything
from thirty to three hundred have beensuggested. The writer suggests thatthe modest figure of a hundred andtwenty will be the optimum number forquite a long time. Such a number willgive very fair reproduction ofan objectwhose linear dimension in the directionof line 'traverse is ,loth of the linearmeasurement of the whole picture inthe same direction, while in certaincases Thth may be similarly repro-duced.
It would be very nice to be able toreproduce all that one can see bycareful inspection of a cinema film, butis such perfection really necessary ?If you watch a movie programme rathermore carefully than usual you will atonce realize that it is very rarelynecessary to see detail finer than thatwhich has an area bounded by 51,th ofthe breadth and height of the picture inorder to interpret all that the producerintended. It is no argument to saythat because one can see the detailedmarkings of, say, a sparrow at thebottom of one's garden some twentyyards away as well as seeing the wholeof the garden at the same time thattherefore television must have the sameresolving power ; even the cinemawould not attempt such a " shot." Ofcourse, the eye only sees very little ofthe total picture area recorded in any-thing like fine detail and as in presentinga scene in the cinema picture, thoseresponsible fill the picture area ade-quately with what they want to " putover " so a similar course would' befollowed in television.
With regard to the direction ofscanning, that of horizontal appears tosuit best the greatest number of sceneswith the exception of the human face.As already shown, detail at right anglesto spot traverse reproduces best andundoubtedly if a square picture wasalternatively scanned horizontally andvertically the greatest amount of detailwould be transmitted for a givennumber of lines. Unfortunately sucha system of scanning would tend toincrease flicker for a given number ofpicture scans per second.
While on the subject of flicker theproblem is of rather greater magnitudethan in cinematography owing to thenature of the television picture. It hasbeen shown that to eliminate flickerentirely in a well lighted picture, forty-eight pictures per second will be neces-sary. The television engineer, how-ever, has great hopes that such picturespeeds will not be necessary and thatthe effect known as " after -glow " offluorescent substances will help to solvethe problem.
24
JANUARY, 1934lailY1SIOn
The Miraco KitTELEVISION FOR THE HOME -CONSTRUCTOR
ALTHOUGH the prices of tele-vision components have drop-ped very considerably during
the last few months, few readers knowthat it is now possible to buy a com-plete kit of components, includingvalves, for a high-class mirror -drumtelevision receiver for £18 as.
Grafton Radio, Ltd., have just intro-duced their Miraco kit, designed byC. P. Hall, a well-known contributorto TELEVISION, which does enable thehome constructor to receive the B.B.C.television transmissions with excellentquality.
This kit of parts is capable of beingassembled by anyone with a few toolsat his disposal. It is supplied in two
parts, the first comprising a very up-to-date mirror -drum visor with anoptical system as a separate unit ; andthe second the radio receiver, whichembodies all the most modern ideas.The tuning is comparatively "sharp, butnot sufficiently so as to cause frequencycut-off and consequently bad pictures.It consists of two high -frequencyamplifying stages, using screen -gridvalves, a combined diode detector andscreen -grid low -frequency amplifierwith a pentode output valve giving
thirteen watts. The whole receiver isentirely A.C. mains operated, quite freefrom hum and extremely compact.
The optical section is mounteddirectly above the vision receiver. Itutilises a mirror -drum scanner of con-ventional design with a Kerr cell andoo-watt projection lamp. The drum
is supplied already assembled and ad-justed so that the difficult part of theequipment is already done for you.The most inexperienced amateur cannotfail to obtain satisfactory results withthis simple equipment. Blueprints aresupplied so that both the sound receiverand the optical portion can be assem-bled without difficulty ; the wiring isshown on the full-size blueprint.
It is quite an easy matter to test outthis equipment We suggest that yougo about it in the following way :
The picture on the leftshows the mirror -drumscanner with Kerrcell and projectionlamp, and the visionreceiver ready forplacing in the cabinet.A photograph of the re-ceiver and amplifier isshown below.
Connect a 2-microfarad condenser inseries with the loudspeaker and coupleit to the output terminals marked 2and 3. The other two terminalsmarked 4 and 5 should be shortedtogether. The tuning condensersshould be adjusted until you hear thelocal station with the volume controlabout three-quarters of the way on.The main tuning condenser is supplied
BRIEF SPECIFICATIONMakers : Grafton Radio Company,Limited.Model : The Miraco Kits to thedesign of C. P. Hall.Price :Valve Combination : Two screen -grid high -frequency stages, diodetetrode second detector, pentodeoutput, full -wave valve rectifier.Type : Table cabinet.Remarks : This is one of the firstcomplete television kits that iswithin the financial and technicalreach of the home constructor.
with trimmers. These should then beadjusted until you obtain the maximumsignal strength. As these trimmers arerather sensitive, the best position willbe found by ear quite simply.
Then tune the radio receiver to theNational programme on 261 metres,from which the television broadcastsare sent out. The condenser and loud-speaker should then be disconnected,and the Kerr cell terminals on theoptical unit connected to the two ter-minals 2 and 3. The two terminals onthe synchroniser should be joined toterminals 4 and 5 on the receiver,which should now be uncoupled.
Switch on the receiver and lamp, andyou will see a yellow beam through the
Kerr cell and on the mirror on the frontof the optical unit. The screen shouldbe pulled forward on the slide about6 in., and the mirror then adjusted sothat the reflected beam passes throughthe lens on to the mirror drum fromwhence it is reflected to the screen.Then revolve the drum, taking care notto handle the mirrors, which wouldspoil the picture -definition and adjustthe forward mirror very finely until thethirty lines scan the screen. The
(Continued at foot of next page.)
25
16;11 71VISIOn
AnJANUARY, 1934
Experimenters NotesAn AmplifierTest
pLAYING round with a cathode-ray receiver the other day Imade an interesting discovery.
Owing to the limited hours in whichtelevision is available at the moment,one gets into the habit of testing anychanges in the circuit on ordinarybroadcast music or speech. Music, ofcourse, is better because it is made upof more or less sustained notes, and ifyour cathode-ray tube is already build-ing up a series of 3o lines, the modula-tion of these lines by the music willcause them to go light and dark inpatches, and the result is a series ofconstantly changing but still welldefined patterns.
The definition of these patterns issome guide to the manner in which theoutfit is performing, and I was rathertroubled by a somewhat faint butnevertheless clearly defined pattern
which appeared to be present as a back-ground the whole time. The patternwas a very fine one, giving the lines anappearance of watered silk. I spent alittle time looking round for troublesin the receiver, but when I thoughtabout it, it was clear that a pattern ofthis nature could only be produced bya very high oscillation.
This gave me the clue. I was listen-ing to the music on a pair of headphonesand looking at the pattern at the sametime. The background to which Ihave referred was produced by a veryhigh-pitched heterodyne whistle (x x,000
3/6 per Quarter6/9 per Half -Year13/6 per Annum
will ensure " Television "being delivered to you regu-
larly each month.
cycles) between London Regional andMuhlacker which was quite inaudibleon my headphones. I replaced themby another pair which I knew had bettercharacteristics, and the whistle at oncebecame audible, although only justbecause it was extremely high andpractically at the limit of audibility.Yet the cathode-ray tube was showingit up beautifully, indicating that myamplifier was working really well froma television point of view. Also itthrows an interesting sidelight on theextremely high frequencies which mustbe handled satisfactorily by a televisionamplifier if the detail is to be portrayedsatisfactorily.
BAIRD TELEVISION, LTD.-CHANGE OFADDRESS.
Baird Television, Ltd., inform usthat the administration office of thisCompany has been removed to 58, Vic-toria Street, London, S.W. x . Tele-phone, Victoria 7238.
The Miraco Kit.(Continued from preceding page)
receiver volume control should thenbe turned up gradually until black linesand patterns appear on the screen. Ifby any chance orange colours appear,the volume control should be turneddown a little, otherwise the Kerr cellmay be damaged.
About five minutes before the tele-vision broadcasts start, switch on themotor to let it warm up, otherwise thespeed may be irregular for the first fewminutes. Take care that you do notswitch on the lamp, or the Kerr cellmay get hot and deteriorate. Theresistance that controls the speed ofthe motor should be adjusted until youobtain the illusion that the spokes ofthe drum are stationary against theglow of the neon lamp. When thetelevision broadcast signals appear thesynchroniser will automatically adjustand regulate the speed of the drum.
The whole equipment looks verycomplicated, but let us assure you thatit is far from being so. It has beendesigned so that the home constructorhas little or no difficulty in obtainingtelevision reception. The blueprintand constructional details have beenso clearly worded that we feel sure the
merest novice will be able to boast thathe has made his own television re-ceiver.
Think of what it will mean to youlater in the year when the B.B.C. startbroadcasting illustrated news bulletins.At the moment the programmes areextremely good. Some of the vaude-ville items are of the highest standard.
The Constructors' CircleAdditional Members
BOYLE, GEORGE, 20, Sandford Road,East Ham, E.6.
REYNOLDS, H., 3, Oak Street, Black-wood, Mon.
BALDWIN, A. R., Spring Hill, Nails-worth, Glos.
Coox, N., 8, Orange Street, Canter-bury.
A RTER, DOUGLAS, The Nest, WarrenAvenue, Charlton.
The Television SocietyThe next meeting of the Television
Society will be held on January 17,1934, at 7 p.m., at University College,W.C., the subject being Cathode- rayTelevision, with a demonstration. Par-ticulars of the Society and proposal
forms can be had on application to themembers' Hon. Secretary, J. J. Denton,25, Lisburne Road, N.W.3.
Television LecturesA series of twelve lectures will be
given by J. J. Denton (Fellow Brit.Radio Inst., Hon. Sec. TelevisionSociety) on Fridays from 7.3o to9.3o p.m. at Morley College, 61, West-minster Bridge Road, S.E.x, com-mencing January I. The fee is only4s. 6d. for the whole course, and thelectures will be illustrated by experi-ments, lantern slides and demonstra-tions.
Television Conference in MoscowAn All -Union conference on tele-
vision is taking place in Moscow. Theconference is dealing with future re-search work to be carried out in tele-vision and the production of trans-mitters and receivers by the Sovietelectrical industry. Mr. Shostakovich,of the Commissariat for Posts andTelegraphs, who has returned from atrip to the United States, is reportingon the progress made in television inforeign countries.
26
JANUARY, 1934
Selectivity or Frequency Response ?In this article S. RUTHERFORD WILKINS discusses a method of retaining the higher frequencies so
necessary to television reception, and at the same time avoiding interference from neighbouring transmitters.
THE design of a television receiverand that of a broadcast receiverdiffer considerably as regards
the high frequency amplifier., In a broadcast receiver, selectivityis the main consideration, but with thetuning system arranged to give a highdegree of selectivity, a considerableattenuation of frequencies above 3,000cycles is inevitable. This is not a very
picture has quite a good definition.In order to avoid attenuation of these
higher frequencies, it has been usualto design a receiver with one or twostages of high frequency amplificationwith flatly tuned circuits. Admittedly,if the tuning is flat enough, very littleattention of sidebands up to to kilo-cycles will result. In order to achievethis, however, the skirt of the tuning
SIMPLE.TUNEDCIRCUIT
SIMPLE BANDPASSFILTER
DOUBLEBArlDP455FILTER
Fig. 1. Curves showing the relative frequency bands passedby various types of tuning circuits.
serious fault in a broadcast receiver,as with most speakers reproduction ofthese higher frequencies is very poor.
WideFrequency Band
The range of frequencies requiredfor good picture reproduction is muchgreater, however, as the degree ofdefinition of the picture depends on thefrequency range of the modulation.In the new multi -line short-wave trans-mission the highest modulation fre-quency transmitted runs into hundredsof kilocycles and corresponds, in fact,to the frequency band of the longerbroadcast wavelengths. Such modu-lations frequencies obviously would beimpossible when the transmissionsthemselves are taking place on broad-cast wavelengths.
As a matter of fact, the frequencyband is limited to to,000 cycles withthe 3o -line transmissions which areradiated from the London Nationaltransmitter, and if the full frequencyband is properly received the resulting
In order to.t
achieve a uniform re-sponse up to o,000 cycles, it is reallynecessary to have bandpass filters witha peak separation of i 2 kilocycles.This can be quite easily accomplishedby slightly altering the value of thecoupling between the two circuits ofthe bandpass filter.
If capacity coupling is employed, itis only really necessary slightly to
0OEEE I
-011T+
Fig. 2. Experimental circuit of double bandpass high -frequency stagefor television reception.
peak will be so wide that interferencefrom stations as far removed as 8o ortoo kilocycles will be noticed.
Selectivity ofTuning Circuits
It is as essential in television as inradio to avoid appreciable interferencefrom neighbouring broadcasting sta-tions, otherwise unwanted modulationwill be present with consequent distor-tion of the image. Thus a televisionreceiver has to have the two ratheropposing properties of reasonably highselectivity and a frequency response upto to kilocycles.
The solution that immediatelysprings to one's mind is to employbandpass tuning. Unfortunately, how-ever, most modern bandpass coils aredesigned to give approximately 8-9kilocycles separation at the peak of thetuning curve. This means that, apartfrom all other sources of loss, therewill be definite attenuation above8,000 cycles in the low frequencymodulation output.
decrease the value of the couplingcondenser.
With an input filter of this descrip-tion it would be quite possible to obtaina uniform low frequency output fromthe set up to to kilocycles, providedthat the other tuning circuits werefairly flat.
Unfortunately, a simple bandpassfilter, although better than a singlecircuit arrangement, does not give atuning curve with really steep sides (seeFig. 1). For instance, a bandpass filterdesigned to give a peak separation of
kilocycles would pass quite appre-ciable interference from a broadcasttransmitter separated 30-40 kilocyclesfrom the one being received.
The IdealBandpass Filter
The ideal bandpass filter, of course,would be one that has the same separa-tion at the bottom of its response curveas at the top.
The goodness of a filter from thispoint of view depends on the number
27
JANUARY, 1934
of circuits included in it. For instance,a '2 kilocycle bandpass filter with fourtuned circuits would give a very muchsteeper response curve than the morecommon type with two circuits only.Unfortunately, it would be expensiveas well as impracticable to include sucha filter as this in a simple receiver.Practically the same effect could beachieved, however, by two two -circuitfilters, one on the input to the high-frequency valve and the other in theanode circuit of the valve.
DoubleBandpass Tuning
For instance, a receiver with oneefficient high -frequency stage anddouble i 2. kilocycle bandpass tuningcould be relied upon to give adequatelow -frequency response at io,000 cyclesand at the same time should obviateserious interference between stations
separated by I5 -2o kilocycles. Althoughthis is not an extremely high degree ofselectivity, it should be quite sufficientto enable the National programme tobe received quite free of interferenceon a set with a single H.F. stage.
Having designed an efficient high-frequency stage for our set, the nextproblem is the rectifier.
Any form of retroaction would beuseless as, besides introducing distor-tion, it would upset the matching ofthe bandpass filter immediately pre-ceding it. Power -grid, or poweranode -bend rectification, would giveexcellent results if carefully adjusted,but from the point of view of sim-plicity and freedom from distortionanode rectification is undoubtedlysuperior.
Use can then be made of one of thenew double -diode -triode valves, toperform the dual function of rectifierand first L.F. amplifier.
A circuit showing the high -frequency
and detector stages of a set on theselines is shown by Fig. z.
It will be noticed that the diode istapped down the secondary coil of thesecond bandpass filter to reduce theload on this coil and prevent mis-matching.
The output from the triode shouldnow be taken to an amplifier of suitablefrequency characteristic, the number ofvalves in the amplifier being dependenton the type of visor in use. For in-stance, if a mirror -drum apparatus isbeing used, the amplifier will have tobe capable of producing 5 watts un-distorted output with at least 500 voltshigh-tension. If a cathode-ray receiveris in use, however, the output fromthe duo -diode -triode should be enoughto modulate the picture.
In any case, the design of a suitableamplifier is quite straightforward, anda high -frequency amplifier designed onthe lines detailed above should give asuitable output.
EUROPEAN TRANSMISSIONS
Transmitter. Wavelength. Power. No. ofLines.
No. ofFrames.Second.
PictureRatio.
Transmission Times.G.M.T.
Remarks.
Baird U.S.W.from CrystalPalace 6.o5 5 M. 3o VariousB.B.C. U.S.W.from Broad-casting House 7.75 m. 120 VariousBerlin ... 1,635 M. 6o kw. 3o 12.5 3-4 Tuesdays, 8.5 to 9 a.m.
(183.5 kc.) Thursdays, 12.45 to 1.45 p.m.Saturdays, 8.5 to 9.45 a.m.Berlin, U.S.W. 6.985 m. 4 kw. 90 2.5 5-6 Daily, 9 to To a.m., excluding
(42,950 mg.) Sundays and holidays. Oc-casionally i to a p.m. and8 to 9 p.m.Leningrad, 857.1 M. 100 kw. 3o . 12 3-4 Irregular.R.V. 53 (35o kc.)
London 261.6 m. 5o kw. 3o I 21 7-3 Mondays, Tuesdays, Wednes-.National (11.47 kc.) days and Fridays, 11 to 11.30p.m.Moscow ... i,000 m. ioo kw. Every other day (uneven dates
(300 kc.) in December), 9.15 to iop.m.
Telecinema trans -missions.Rome ... 8o m. 9 kw. 6o zo 4-3 Discontinued at present. Commencing(3,75o kc.) again soon.25.4 M. 9 kw. Special an-(ii,810 kc.) nouncemeniwill `be made.
PLACE AN ORDER WITH YOUR NEWSAGENT FOR ," TELEVISION " TOBE DELIVERED EACH MONTH
28
JANUARY, I (); ; 1",'LP/ISIO1
A VALVE RELAYAnd Some Typical Applications
The gas -filled relay has become an essential part of the modern Cathode-ray televisionequipment. Here are some useful facts concerning the Osram mercury -vapour tube.
THE Osram gas -filled relay typeG.T.i is a gas -discharge valveconsisting of a cathode, anode
and grid in a mercury -vapour filledbulb.
The electron emission is obtained
When the discharge is in progress thevoltage drop across the gas -filled relayremains constant at about 15 volts,independent of the load.
One important point in the operationof the tube is that the cathode should
LOAD+ H.T.
OSRAMGAS -FILLED RELAY
G.T.I.
H.T.
4 VOLTS
A.C. OR D.C.The gas -discharge tube used as a simple relay with D.C.
anode voltage and D.C. grid control.
from a coated cathode indirectly heatedby a 5.z watt heater which it encloses.This has the advantage, in combinationwith the standard heater rating of4 volts, that the filament may be heatedfrom a transformer common to other
relay this ratio is approximately z 5 :
that is, for example, a negative gridvoltage of to will suffice to withholdthe discharge up to an anode voltageof 15o (the discharge in any case notcommencing under 15 volts).
CONTROL
An inverter circuit-D.C. to A.C. with D.C. anodevoltage and A.C. grid control.
be allowed time to attain its full oper-ating temperature (at least r minute)before the anode voltage is applied.Failure to observe this precaution willresult in permanent damage to thecathode. For the first time of switch-
ing on a greatercathode heatingtime (5 minutes)should be allowedin order to ensurecorrect distributionof the mercury in thebulb.
The function ofthe grid in theOsram gas -filled re-lay is to controlthe anode voltageat which the dis-charge commences.A negative voltageapplied to thegrid will preventthe discharge
being established, its minimum valuedepending upon the applied anodevoltage and the " grid control ratio."In the case of the type G.T. r gas -filled
OSRAMPHOTOCELL
6 A.C.A typical application of a gas -filled relay working in
conjunction with a photo cell.
indirectly -heated valves if desired.Under normal operating conditions thebulb is filled with a blue glow, due toionisation of the mercury vapour.
The moderately low value of controlratio avoids a highly critical operatingcondition. The actual grid controlratio depends on the temperature of thesurrounding air and of the gas -filledrelay. A reduction in temperaturewill increase the control ratio-i.e., fora given anode voltage a smaller nega-tive grid voltage suffices to withholdthe discharge. After the dischargecommences the grid exercises nofurther control, and the anode currentmay in general only be stopped bybreaking the anode circuit or reducingthe anode voltage to a condition belowthe ionisation voltage-normally aboutI 5 volts.
The time required for the grid toregain control, while the discharge isoff, is extremely small, a fraction of amilli -second.
It is important to note that when theanode current is once started its valueis limited only by the external impe-dance in the anode circuit. To avoidinjury to the valve it is essential thatanode current shall not exceed the ratedpeak value of o.6 ampere. To thisend a suitable resistance, or other
9
JANUARY, 1934
current limiting device, must alwaysbe included in the anode circuit. Forexample, in the case of a resistance load,if the D.C. voltage is zoo, and a 15 -voltdrop in the gas -filled relay is allowedfor, this resistance may be obtained asfollows :-
Maximum permissible peak current-o.6 amp.
Volts drop required across resistance-185.
185Resistance =3o8 ohms.
For A.C. voltages the anode resist-
ance must be calculated for the peakvalue of the voltage-i.e.,
\/z X R.M.S. value.Thus, if the A.C. voltage is zoo
z
6.200)-I5Resistance -( -448 ohm
If a tungsten filament lamp is con-nected in the 'anode circuit, it shouldbe noted that the cold resistance ofsuch a lamp is about one -fourteenth ofthe resistance when hot, and conse-quently an excessively heavy currentwill flow when first switching on beforethe tungsten filament becomes hot.
It is advisable to operate with aresistance (about Io,000 ohms) inseries with the grid to limit the gridcurrent for positive grid voltages.This resistance does not appreciablyaffect control when the grid is negative.With A.C. anode voltage and A.C. gridcontrol, continuous control of anodecurrent, from zero to maximum loadcurrents, may be obtained by variationin phase angle between anode and gridvoltages. This is of considerable im-portance in all cases where continuouscontrol is required.
PATENTS AND PROGRESS-Continued from page 14is projected directly on to the "mosaic"surface, where it is immediately con-verted into an equivalent " electrical "picture, the high lights setting -up largeelectrical charges and the low lightssmall electrical charges. At this stagethe picture is scanned by the cathoderay and the small cell -condensers aredischarged as a series of varyingcurrents.
In order to allow the picture to beprojected on to the " mosaic " fromoutside the cell, the anode of thecathode-ray tube must be set at anangle to its usual position. Unfor-tunately this means thatforeshortened with respect to thecathode ray, a fact which is liable toproduce a certain amount of distortion.To prevent this the picture is projected
on to the mosaic of cells through aspecial system of lenses which slightlydistorts the picture in the reverse senseto that previously described, the resultbeing that one deformation offsets theother, and so produces a true image atthe receiving end. - (Electrical andMusical Industries Ltd. and W. D.Wright)
Other Television Patents(Patent No. 399469.)
Velocity -modulated television sys-tem in which the deflecting potentialsfor the cathode-ray tube are derivedfrom a condenser controlled by ther-mionic valves.-(W. R. Bullimore andL. H. Bedford.)
(Patent No. 399694.)Dynatron valve used as a television
(Patent No. 400062.)Magnetising coils for focussing the
electron jet in a cathode ray tube.-(Telefunken Co.)
(Patent No. 400453.)Cathode - ray tube for television in
which a " throttling " diaphragm isplaced in front of the cathode tocontrol the stream.-(Telefunken Co.)
(Patent No. 400610.)Synchronising systems for television
in which a driving motor is automa-tically checked from running out oftime.-(Marconi's Wireless Telegraph Co.Ltd., H. M. Dowsett, and L. E. Q.Walker.)
THE SCOPHONY SYSTEMThe Editor is accorded the first pressdemonstration and an examination of theapparatus.
IN photography the word " kodak "has become synonymous with
simplicity. It would appear that theword "Scophony " may come to havethe same meaning in television, forsurely no mechanical television appar-atus could be more simple or requireless power for its operation than thatwhich has this appellation. The sim-plicity, however, is in the completeapparatus ; the principle and construc-tion are extremely complicated, thelatter embodying a great deal of fineoptical work.
We are not at liberty to disclose theactual details yet, beyond saying thatthe principal unit in the apparatus is arevolving echelon which in the smallmodel weighs but a few ounces and,being perfectly balanced, requires
negligible power to drive. There areno other moving parts whatever.
At present a line mercury vapourlamp is being employed as the illu-minant, but other sources of illumina-tion are equally suitable. This type oflamp requires the minimum amount ofpower for its operation, and this,combined with the high efficiency ofthe echelon, accounts for the extremelysmall power required to operate theScophony receiver.
Photographs on other pages in thisissue show the two types of receiverdemonstrated. One, it will be ob-served, is but little larger than a handcamera, and yet it will project pictures3i in. by in. The extremely smallsize of the echelon scanning devicewhich is used with this receiver willalso be apparent from this photograph.
The other receiver is the standardinstrument, and this is built into ametal framework ; the simplicity of
this is also apparent from a photo-graph. At the demonstration wit-nessed the results were about the equalof those obtained with high-classmirror -drum apparatus, but with theadded advantage that there was analmost complete absence of scanning-line demarcation. Also it must beremembered that the input was lessthan ordinarily used for mirror -drumreproduction. The lighting of thepicture was all that could be desired.
The reason why this apparatus is notavailable to the public is explained byMr. Sagall on another page. Exam-ination of the Scophony receiver makesit quite evident that it is a type whichcan only be manufactured commerciallyand, if the price is to be kept to the lowfigure intended, on a large scale.
The Scophony system lends itselfequally well to high and low -definitiontransmissions. Fully illustrated detailswill be given in an early issue of thisjournal.-H. C.
30
JANUARY, 1934
A Popular Disc ReceiverThe completereceiver with-
out cabinet.
THE photographs and drawingson this page show a simple discreceiver which has attained very
great popularity. Originally it wasdescribed in our sister journal AmateurWireless, and it is felt that brief par -
titulars will be appreciated by readersof TELEVISION who are desirous ofmaking up a receiver of this type.
Three factors were kept in mindwhen this receiver was designed-efficiency, a good appearance and easeof construction-all of which havebeen amply fulfilled. Excellent reportshave been received of the performance,mostly operated from ordinary stan-dard three -valve broadcast receivers,which provide an adequate input undermost conditions.
The appearance of the finished re-ceiver can be judged from the photo-graph, and it will be seen that it is quite
neat. Special attention was given tomake its construction come within theabilities of the average amateur whoonly has the simplest tools available.The receiver does, in fact, represent thesimplest and cheapest construction con-sistent with really good results. If itis not wished to make the woodwork,this can be obtained ready fromMessrs. Peto-Scott, Ltd. All the otherparts are available from the MervynSound and Vision Co., Ltd.
It will be seen that the main parts ofthe receiver are the scanning disc(which is of small size and was speciallydesigned for this machine), the motor,
The ComponentsOne scanning disc ready for use (Mervyn Sound and
Vision Co., Ltd.).One motor, type BRI (Mervyn Sound and Vision Co.,
Ltd.).One 41 in. diameter lens (H. Sanders and Co.). One
small lens.
You Will NeedOne beehive or spiral neon lamp (G.E.C., Philips).One terminal mount, type " B " (Belling -Lee).One 3 go -ohm variable resistance to carry .3 ampere
(Mervyn).One mains plug.Baseboard and woodwork (Peto-Scott).
OPENING
ex 3jp
LENS4DI AM
34-/
- PLYW
r3.
2"
5 -NN> 41/2
34Here are details of the cabinet, lase -
board and rheostat mount.
the neon lamp (of the beehivepattern), two lenses and themotor -control rheostat. Thesmall lens is a sixpenny
OPENING reading glass.134x154
LENS2 Y.;
DIAM.
A large-scale blue-print of the com-plete set of parts isavailable, price if-.
20'..-"\-MAINS PLUG TERMINAL STRIP
y` Ida9 -HOLES-AT V:
-
The photograph on the right shows thecomplete receiver ; on the left is adimensioned drawing of the lens mount.
31
lar..v1S.10flJANUARY, 1934
How to Receive the Broadcasting House andCrystal Palace Short-wave Transmissions
Some General Hints on Ultra -short Wave Receptions
THE first people to put the ultra -short waves to practical usewere the Germans with their
7.5 -metre station at Berlin. As thisstation was so successful, having aservice area of up to 75 miles or so,these low waves began to occupy theattention of the whole world's radio
70 V
7 MMFD.
3$ mmFD..0003 mFo
H.T. I -
90V.
H.F.C. 0005 MFD.
A
LT.-
LT+
Hook up this unit and pick up television foryourselves. Note the low capacity of the tuning
and aerial series condensers.
engineers. From the very early daysof television it has been quite clear thatthe broadcast wavelengths would be ofvery little use if a high degree of defini-tion were to be obtained. After con-siderable data had been collected aboutthe funny little ways of these quasi-optical waves, television was broadcastfrom Berlin every day. These trans-missions were so successful that beforelong receivers were being installed atdistances up to 75 miles from thetransmitter. That the use of televisionreceivers did not become universal wasdue mainly to the high initial cost, forremember that an hour or so's broad-cast each day is not very much for anexpenditure of between £30 to £40.
Round about this time Baird wasexperimenting at his Long Acre studiowith various methods of transmissionon a wavelength of 6.1 metres. Thathis experiments were successful areproved by his latest broadcasts fromThe Tower at the Crystal Palace.
In America some twenty commercialstations were soon on the air, using
By Kenneth Jowersdifferent transmitting systems. It wassoon quite obvious that to present anation-wide television broadcast a chainsystem of television is essential-thatis, the linking up of a number of low -power transmitters which betweenthem would cover a very considerablearea.
To -day plans are almost completedin America for the transmission oftelevision on this chain system, thevision being transmitted on wave-lengths between 7 and 8 metres, andsound on 4.5 to 5 metres.
In Germany the chain idea hasalready been put into action, wherethey have decided to erect twenty orso stations in or around the large townsand densely populated areas.
These stations will all radiate thesame programmes and be synchronised,so that television will be on tap foralmost all of the German people withthe necessary receiving equipment.Naturally the cost is still a trifle high,but it is dropping very rapidly.
Short-waveReceivers
Do you realise that a 7 -metre set isthe easiest thing in the world to make,if you know how. If cost is a stum-bling block, why not make a simpleunit to work in conjunction with yourpresent set ? It does not matter if youcannot afford a vision receiver-seewhat you can do with a sound receiver.Experiment with these fascinating shortwaves and be the first to hear the7 -metre programmes in your district.
Before anyone knew very muchabout these quasi -optical waves, it wasconsidered that the maximum usefulrange or service area was in the regionof to to 15 miles. This idea has putoff many amateurs from building theirown sets, thinking that they were outof the range of the nearest station.Well, the reception record has gone upand up until now it stands at zoo miles,and this with a two -valve set.
Television and 7 -metre sets are forthe home constructor-for the man
who likes making his own components-because they are so easy to make andcheap. Take the coils as an example.Merely two turns of a heavy gaugecopper wire, wound on something likea broom handle. That is the grid coil.The reaction coil requires three turns.
SuitableCircuits
On the other hand, if you wish tobuy these components, there are manu-facturers who specialise in the coils andcondensers that you want. The mostdifficult job is to decide on the best typeof circuit to use without trying all ofthem, which would make it expensive.
Follow this photograph when making your 7 -metrecoils, and be very careful about the spa -ing tetneen the
turns as small variations will upset the tuning.
Luckily that trouble is soon overcome,for we have been experimenting withvarious types of circuit for a long time.
We have a laboratory 4o miles northof London-an ideal spot for tele-vision. Being a long way out of theofficial service area, our receivers hadto be good, so you can be sure thatwhen you make up your 7 -metre setor unit you will be well repaid.
32
JANUARY, 1934 .11.11VISMI
Except for those within a mile or soof a transmitter, the straight type of setmust be ruled out. The popular re-acting detector circuit is of little use,even if you can get the reaction smooth.At the/moment the super -regenerativecircuit does not give sufficiently goodquality, although it is good enough forsound or for finding out if you are inthe service area.
The only circuit that has stood thetest of time is the superhet, but eventhis circuit must be modified for theseultra -short wavelengths. If you haveal broadcast superhet, you will be fullof trouble should you try to use a con-verter in front of it. Contrary to whatyou probably expected, selectivity isnot wanted ; the flatter the tuning thebetter.
Take a look at Fig. r : quite simple,isn't it ? A compromise between atheoretical and a practical diagram.At first glance you might think that itis a conventional screen -grid detectorcircuit ; so it is, but with a few reserva-tions. Starting at the beginning of allthings, the aerial, the first componentis a 7-micromicrofard variable con-denser, not many of these in the junkbox. Anyway, Strattons have plentyof them, as well as the 3 5-micromicro-farad tuning and xoo-micromicrofaradreaction condensers. The only otherspecial component you should buy isthe high -frequency choke. Try Igranicor Strattons for this. The valve holdershould be of low capacity, which youmay have, otherwise Strattons again.
Coil -making is an easy job, but incase of accidents we have given asimple diagram. Obtain about z ft.
of 14 -gauge bare copper wire andstretch it to get out all the kinks. Thenfind a r -in. former-our broom -handlewas the exact size. Cut the wire inhalf and wind two coils on the r -in.former. The first one, the grid coil,consists of two complete turns, andthe second, the reaction coil, of threecomplete turns. The spacing betweenthe turns should be about the same asthe thickness of the wire. If fairly longends are left, say about ri in., the coilscan be connected to the proper pointswithout any holder. The grid coil isa good example of this, for it can beconnected directly across the 33-micro-microfarad condenser.
Although hand capacity is not worthworrying, about, it is advisable in caseof accidents to use 6 in. extensionhandles. Ultra -slow-motion tuningdials, such as the Igranic microvernier,will be a great help in tuning.
SimpleConstruction
Constructing this simple unit willnot present any difficulties. It is thereceiver to which it will be added thatmust be looked at.
We have told you before that selec-tivity is not wanted, so it must be gotrid of before anything else is done.Let us consider one of the most popularcircuits of the day, the screen -griddetector pentode combination. If itis coupled to the aerial by means of aband-pass coil, this coil must be cutout. It is not necessary to upset thewiring. You just join the outputterminal of the 7 -metre unit directly to
The light tell of a German daylight transmItting system.
the G terminal of the screen -grid valveholder.
Should there be an adjustable con-denser coupling the screen -grid stage tothe detector stage, screw this down as faras it will go. That's about all you cando, except to make quite sure that thereceiver is quite free from distortion.Connect up the unit in the usual way-high tension plus to about 90 -volts,the A terminal on the unit to the Aterminal on the family set, unless thereis a band-pass coil, and then to the gridof the valve holder. You might noticethat when a common high-tensionbattery is used there is not a secondhigh-tension negative tapping. Shouldthere be the slightest trace of distortionin the output stage, replace the outputvalve with one having a larger gridswing. Quality rather than quantityis the order of the day ; remember acathode-ray tube only requires 3o voltsto modulate.
The BestAerial
All your precautions will be in vainif the aerial system is incorrect. Whenwithin the normal service area, ro orz feet of wire, one end on the aerial
terminal of the set and the other endas high as possible, will be as good asanything.
But when the receiver is zo or 30miles away from a station more caremust be taken. An effective aerial isthe simple T -type, the length beingapproximately the same as that of thewavelength of the station you are goingto pick up. The length need not bevery accurate. An example will showyou just what we mean.
At the present moment the BairdCompany are broadcasting from theCrystal Palace on a wavelength of6.033 metres. This is about zo feet.Erect an aerial of this length, and inthe exact centre tap off the lead-in wire.That gives you half wavelengths eitherside of the lead-in. Make the lead-inwire the same length as the aerial.Then you will be able to erect it about13 to 18 feet high. Any increase inheight or length will probably causedamping of the grid circuit, and theconverter will stop oscillating.
An order placed withyour Newsagent will en-sure regular delivery of
"Television."
33
'flitilY1S1011
Photo -electric Cellsfor Colour
THE art of making talking pictureshas called into being several newtypes of photo -electric cells.
Television in natural colours withwhich experiments are being made hascaused a demand for yet other typesof these cells. The success which hasattended the physicists' efforts to evolvethese new cells is remarkable, and tendsto show how surely all the many intri-cate problems connected with televisionwill be overcome in due time.
Natural -colour television, dependingas it must for the time being on theprinciples of three -primary colourvision, it is, of course, a sine qua non thata transmission must involve the useof three wave -bands, the blue -violet,the green, and the red components ofthe image being each transmitted on itsown wavelength. The alternativemethod-to send blue -violet, green,and red signals after each other incycles-has failed completely in colourcinematography (on account of colour" fringes "), and is hardly likely tosucceed in television.
SplittingUp the Image
The need for splitting up the imageto be transmitted into its blue -violet,green ancl, red components sounds adifficult matter at first, but it has nowbeen accomplished in an amazinglysimple manner by the invention of newtypes of photo -electric cells, which aresensitive to different colours them-selves.
Just as photographic films have hadto be made sensitive to differentcolours for the purpose of naturalcolour photography, so colour -sen-sitive cells have been produced.
One well-known type of photo -elec-tric cell consists of a vacuum bulb, oneside of which is coated with potassium,in the centre of the bulb there being aring anode which collects the electronsthrown off by the potassium on itsbeing illuminated. The potassium isconverted by hydrogen into potassiumhydride for greater sensitivity, andwhere instantaneous response to lightis not the first consideration, the resis-tance of the bulb is reduced by theintroduction of a trace of inert gas.
A cell has been developed in whichsodium is substituted for potassium,
and its active surface is vastly increasedin sensitiveness to light by a processinvolving the use of sulphur vapourand oxygen, instead of the glow dis-charge of hydrogen.
Photo -electric cells made in this wayrespond to the entire spectrum rangeof colour-violet, blue, green, yellow,orange and red.
The BellSystem
The Bell system of television, towhich these cells have been applied,employs a scanning disc which throwsan intense spot of light on the face ofthe sitter or scene being " televised."The spot of light traces a path over theentire area of the subject every one -eighteenth part of a second, and thelight from the subject is reflected backupon the photo -electric cells.
Now, in photography it has neverbeen possible to make a film quiteevenly sensitive to all colours ; the filmmay be, for instance, twice as sensitiveto red as it is to green, and ten times assensitive to blue. The same troublehas been found with these new photo-electric cells. As a result it is necessarythat the light reflected from the sitter'sface (or other subject) be cast upon anarrangement consisting of two cellsprovided with blue -violet screens or" filters," eight cells with green filters,
JANUARY, 1934
Televisionand fourteen cells with red filters. Inthis way, colour equilibrium is obtained,the blue, green, and red cells beingrespectively coupled up in parallel, andthe current from each set after amplifi-cation being simultaneously transmittedon its own wavelength.
Three sets of television signals arethus received at the viewing station,representing the three primary colourcomponents of the coloured subjectbeing televised. Each series of signalsis made to actuate one of three lightsources, which must in turn produceflashes of blue -violet, green, and redlight respectively. The scanning discthrough which the observer's eye isdirected is, of course, in synchronismwith the transmitting disc and re-combines the three primary colourimages into a single image in naturalcolours.
But here again difficulty was encoun-tered in the fact that the ordinary neonlamp does not emit blue. and greenrays of light. As all who haveseen the neon lamp can appreciate, thereis no difficulty about the red. Theneon glow is very rich in red rays, andby placing in front of it a suitable redglass screen or filter, pure red andorange rays only will be obtained. Toproduce light sufficiently rich in greenand blue rays, argon lamps are em-ployed.
The Mihaly stationary mirror drum Receiver. In the Mihaly 'apparatus the mirror drum isstationary, the modulated light passing underneath this on the lower part of the revolving mirror
in the centre of the mirrors.
JANUARY, 1934
principal media which can be used ina Kerr cell (though it must be remarkedthat most liquids show the Kerr effectto some degree.
For the experimenter, the most con-venient medium to use is nitro -benzene,provided he obtains it of excellentquality, and is careful not to let it comeinto contact with air until it is to beused. No difficulty should be experi-enced in obtaining a plate -to -plateresistance of over a megohm throughthe liquid, and if the resistance fallsbelow this it should be changed.More, however, will be said of this andother practical considerations in thefinal article of this series, which willdeal with the construction of a cell fortelevision purposes.
For best results, the plane of polar-isation of light entering the cell mustbe at 45° to the plane of the plates, asillustrated in Fig. 1a. If the plane isvertical or parallel to the plates, noresult at all will be obtained.
Following a suggestion of Dr. W. D.Wright (Proc. Phys. Soc., Vol. 44,pp. 325, 1932), the plates may besloped to conform to the rays of lightcoming from a condensing lens, in themanner shown in Fig. 1. This givesslightly increased sensitivity to the cell,and also tends to minimise the ob-scuring effect of the plates in the beam.
Limitations ofthe Kerr Effect
We come now to a most importantproperty of the Kerr effect, and onewhich in practice limits its utility some-what ; this is the fact that after acertain voltage (corresponding withthe condition shown in Fig. le) isreached, the cell begins to pass less andless light through the analyser untilzero is again reached. Thereafter,maxima and minima of light passedalternate, as the voltage is increased,indefinitely. The voltage of the firstmaximum sets a limit to the length ofcharacteristic over which the cell canbe operated, and most important of all:this maximum is not the same for light ofdifferent colours : thus, blue and violetare the first colours to reach theirmaximum transmission, and orange andred last. This gives rise to distinctcolouring, when white light is used,as the voltage approaches the highervalues, and finally when the first maxi-mum has just been passed, a character-istic reddish -brown colour is observedin the emergent light. Theoreticalcurves for a typical cell are given inFig. 3.
From this it will be seen that, al-though from some points of view(notably in order to increase the sensi-tivity of the cell) the slope of one of thehigh -voltage " humps " would seembest for operation, yet in practice thiswould mean very poor efficiency owingto the fact that the blue part of thelight would be increasing while the redwas being extinguished (or vice versa).To use monochromatic light (that is,light of only one colour) would bevery inefficient, and a source of verygreat brightness would be required.Also, there is a danger of the mediumbreaking down if too high voltage isemployed.
ChromaticDispersion
The separating out of the componentcolours of white light owing to theunequal effect of the cell upon them istermed " chromatic dispersion."
Next month we shall give someexamples of characteristics of actualcells, which differ in some respects fromthe theoretical one given in Fig. 3.For those who wish to calculate themagnitude of the voltage, or the dis-tance apart of the plates, to obtain agiven effect, the value of the " Kerrconstant " for several media is givenin the table and can be inserted informula given in the appendix.
APPENDIX I.
For a birefringent liquid, KP2= t/Ax-r/Ay
where K is the Kerr constant for theliquid, P is the electric fieldstrength,
Ax and Ay are the wavelengths of thetwo rays in the liquid.
Hence A =---zirK1P2 is the angular phasedifference of the two emergent rays,where 1 is the length of path in theliquid.
If I is the incident plane polarised light -vector :-
1 = a sin wt say, whence :x= --a cos i sin wty=a sin i sin wt
where i is the angle I makes withthe field -direction.
When the electric field is applied to thecell, these become :
x=a cos i sin (wt + P1)y=a sin i sin (wt 6,2)
where Ai- Q2--= A above.
Hence x2 zxy cos A y2
cos2i cos i sin i sin2 i,_assins A
which is, in general, an ellipse asstated in the text.
APPENDIX II.
We can derive an equation for the lightemergent from the analyser :where I' is the intensity of this light
we have :-I' =a1(cos2 i- -sin zi sin z4, sin2 /z)
where 4, is the analyser angle.For(/' =45° and I= -45° (the case dealt
with in the text) we have :I' =a2 sin2 A/z
and since A =1/TICIP2 from Appen-dix I we get :-
I'=a2 sin2(7.ICIP2).Converting this to a form involving the
voltage v on the cell, and puttinga2=i, we have :
L=sin2
where E iscell giving
the potential across thethe first light -maximum.
THE CONSTRUCTORS'CIRCLE
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JANUARY, 1934
The Television SocietyPresident: Sir Ambrose Fleming, M.A., D.Sc.. F.R.S.
Hon. Secretaries : J. J. Denton, A.M.I.E.E., 2 5, Lisburne Road, Hampstead,London, N.W.5. W. G. W. Mitchell, B.Sc., "Lynton," Newbury, Berks.
AMEETING of the Society washeld at University College,London, on December 13, at
7 p.m. Dr. Clarence Tierney,F.R.M.S., chairman, referred to theimportance of members studying thefundamentals of their subject, andwelcomed the lecturer of the evening,Mr. Leonard M. Myers, B.Sc.(Fellow),who had prepared for them an inter-esting paper, with experiments, whichwould be published fully in the Journalof the Society.
Abstract of lecture entitled :-ELECTRO-OPTICS AND TELE-
VISION.The study of electrostatically induced
stress in both solids and liquids is ofthe utmost importance for televisionwork. Up to the present liquids haveenjoyed the greatest popularity, and ithas been found that nitro -benzene,owing to its high Kerr constant, is themost suitable liquid to be employed.The question remains, however, as tothe manner in which the crystallineliquids acts in order to produce theessential retardation.
As far as we can see, when the beamof polarised light enters the nitro-benzene, it is split up into two com-ponents vibrating in mutually perpen-dicular directions. When the stress isapplied, the one component, vibratingparallel to the direction of the lines offorce, is accelerated and the othercomponent is retarded. For this reasonthe liquid under stress is regarded ashaving the characteristics of a negativeuniaxial crystal, of which Iceland sparis an example.
But this assumption is not strictlytrue, for in a negative uniaxial crystalthere is always one direction perpen-dicular to the optic axis in which theordinary ray appears. By the ordinaryray is meant that ray which has thesame velocity as light passing throughthe unstressed medium. In thestressed nitro -benzene this ordinary raydoes not appear for any directionnormal to the field. Now in a negativebiaxial crystal there is one direction forwhich the ray vibrating parallel to theoptic axis is accelerated, and thatvibrating perpendicular to this axis is
retarded. This direction is normal tothe axial plane of the crystal-that is,the plane which contains both theoptic axes.
But again for the direction parallelto the axial plane the ordinary rayappears. Therefore it is not possibleeven to compare the stressed nitro-benzene with a negative biaxial crystal.The character of this stressed liquidmust be, then, of a particular nature,and we can regard its ray surface asbeing a prolate spheroid encased in anoblate spheroid in such a manner thattheir axes of revolution are equal inlength and that they coincide. Thiscommon axis, then, forms the opticaxis of the stressed liquid, and it isparallel to the lines of force.
In any direction in which the lightin the cell travels the oblate spheroidforming the ray surface of the fast raywill be in advance of the surface of theprolate spheroid, the surface of theslower ray. As the fast ray is vibratingparallel to the optic axis, the crystalmust be negative by definition.
UniaxialCrystals
Experiments were performed toindicate the change in character ofuniaxial crystals and isotropic mediawhen subjected to stress. In the caseof isotropic media, glass was taken asan example, as the conversion of thissubstance into a negative uniaxialcrystal was shown when the glass wascompressed. When the glass wassubjected to tensile stress it becamepositive uniaxial in character. Theglass was bent in the polariscope, inwhich was placed a quartz wedge. Itcould be observed that the upper partof the glass strip was in tension andthat the retardation bands advanced ;
therefore this part of the stressed glasswas behaving as a positive crystal.Quartz itself is a positive crystal sothat an advance of retardation as thatoccasioned by the wedge would indi-acte a positive crystal. But for thelower portion of the glass the retarda-tion bands receded, thus showing thatthe glass under compression acted as anegative uniaxial crystal.
The character of stressed uniaxialcrystals was then discussed, and it wasshown that when these crystals weresubjected to stress normal to the opticaxis they became biaxial, having, how-ever, the same sign as formerly. Inorder to illustrate this point by a prac-tical demonstration, the uniaxial crystalwas observed in highly convergentlight. This method gave a picture onthe screen of the isochromatic lines ofthe crystal. By stressing the crystalin the polariscope, it was seen that theisochromatic rings became ellipses,thus demonstrating that the crystal hadbecome biaxial in character.
ElectrostrictionA short discussion of electrostriction
followed, in which it was shown thatit is possible to bring about retardationin the polariscope by this means. Butthe stress set up in media by electro-striction alone are insufficient to illu-minate the field brilliantly with com-paratively low voltages. It is interest-ing to note, however, that the electro-striction formula is identical with theKerr effect formula except in the valueof the constant. The constants for thetwo expressions have widely differentmagnitudes.
From the Kerr effect in liquids atten-tion was drawn to this effect in solids,which had received by no means thesame popularity with the designers ofelectro-optical devices. Particularstress was laid on the employment ofpiezo-electric quartz as a method ofobtaining the desired retardation. Anexperiment was then made with a discof piezo-electric quartz cut in such amanner that it would vibrate in thedirection of its thickness. Both theoptic axis and the third axis of thequartz was in the plane of the smalldisc, but the electric axis was in thedirection of its thickness.
This piece of crystal was held be-tween two solid brass electrodespressed against it, so as to prevent itslipping out when in vibration, and soas to damp the crystal. The crystalwas so dimensioned that it wouldvibrate at the frequency of about 3
megacycles, corresponding to a wave-length of 1 oo metres.
Now, as the optic axis was in theplane of the crystal it was necessary tocompensate for the retardation set upby it in the polariscope, and this wasaccomplished by introducing a secondcrystal of exactly the same thickness.The light in the polariscope thentravelled through both crystals parallelto the direction of their thickness and,therefore, in the crystal which was
38
JANUARY, 1934
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JANUARY, 1934
excited, through the electrodes. Ahole of about in. in diameter wasbored in both electrodes to allow ofthe passage of the light through them.
Effect ofOscillation
When the crystal was at rest thefield of the polariscope was extin-guished, but as soon as the crystal wasput into oscillation, by means of asingle valve oscillator, by virtue of thestresses set up within the crystal, thequartz became biaxial in character inaccordance with what had been previ-ously discussed. This being the case,the field at once became illuminated.The illumination of the field by such amethod is, of course, not constant, butcan be likened to a lamp giving outlight at the frequency of three millioncycles per second. It appears to theeye, therefore, as though the illumina-tion was constant as the frequency isso high.
The important question which nowarises is how shall the vibrations of thecrystal be modulated so that it can beused for television. As the crystalstands alone and undamped, it is im-possible to modulate much higher thana or 3 kilocycles. This is because thefrequency characteristic of the crystalis so peaked. If such a crystal is em-ployed in a receiving circuit, as in thecase of a Stenode Radiostat crystalreceiver, the crystal is undamped sothat the side bands above 2 kilocyclesare so highly attenuated that highaudio -frequency correction must beintroduced. But the crystal can bedamped in the first place by pure
mechanical pressure, and by this meansthe modulation frequency can beincreased.
Of course, the crystal does notvibrate at one frequency only, and thefrequency discussed above is the funda-mental frequency. Crystals can be cutso as to vibrate at a large number offrequencies not always along the direc-tion of the thickness of the crystal.Therefore it appears that the idealcrystal for modulation in television isone cut so that there are a large numberof resonant frequencies within veryshort tuning distance of one another.
Advantages ofCrystal Modulation
The advantage of crystal modulationfor television over the nitrobenzenecell is very great indeed. In the firstplace, there is practically no absorptionof light, which is the case when thelight has to pass through a thick oilyyellow liquid. Secondly, there can beno gradual disintegration of the quartzas there is of the nitro -benzene. Thelatter being a complex organic com-pound which soon becomes disinte-grated, carbon being deposited on theelectrodes and at once reducing theresistance of the cell. No amount ofplating or silvering can prevent thisdisintegration. By suitable electrodes(which may even take the form oftransparent colloidal films or, alterna-tively, a transparent metal film, suchas the film produced by steel), all lightcan pass through the crystals, and thesection of the light beam can be as largeas desired.
As to the question of cost : at
Television has become a subject of world-wide interest and this journalcirculates in all parts of the world. If you are unable to obtain yourcopy locally we will dispatch it regularly each month after receipt of thisform and remittance.
MESSRS. BERNARD JONES PUBLICATIONS, LTD.,58-61, FETTER LANE, LONDON, E.C.4.
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Remittance to the value of enclosed herewith.NameAddress
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" Television " is published on the last Wednesday of each month, bearing date of themonth following.Subscription Rates: Great Britain and Abroad 13'6d. for twelve months, post free ;6/9d. for six months, post free; 3/6d. for three months, post free.
present, as there is but little demand forsuch crystals the cost comes out toabout f4 per pair, but this could bereduced by half, no doubt, if thedemand was adequate. It must beremembered that the crystals have tobe first very carefully selected and thenground and polished to within a wave-length accuracy. (A wavelength isroughly one -thousandth of a milli-metre.)
In passing, mention was made ofsome research into the possibility ofutilising the piezo-electric vibrations ofRochelle salt as a light modulator fortelevision. But it was found, despitethe rather encouraging fact thatRochelle salt had about 16 times thepiezo-electric constant of quartz, thatthe stress optical coefficient was so lowthat retardati-m of any considerablemagnitude could not be brought abouteven when the crystal was mechanicallystressed to its breaking point.
Finally, a novel form of polariscopefor television was suggested. In thispolariscope use was definitely made ofboth the ordinary and the extraordinaryrays of the polariser. In the present-day methods only the extraordinary rayof the (Nicol prism) polariser is used,so that at the outset half the availablelight is wasted. In the polariserdemonstrated as representing thegeneral method to be employed, twoRochon double image prisms wereused. With the first prism alone itwas possible to discern two separatespots of light, but when both were usedthese two spots of light further resolvedthemselves into four spots.
A hearty vote of thanks to Mr.Myers concluded the lecture and dis-cussion.
TelevisionAmplifiers
It is not always necessary to build alarge and expensive amplifier to obtaina high output for a mirror -drum tele-visor. An arrangement that is notvery often used is a variable -mu screen -grid valve as a low -frequency amplifier,R.C. coupled to a valve such as thePP5400. With the correct resistancenetwork a very high stage gain can beobtained while the characteristics of thevariable -mu screen -grid valve are veryuseful, as a silent and smooth volumecontrol can be obtained. With anamplifier of this kind a minimum of5 watts can be obtained without anydifficulty, while if it is compared withthe conventional 5 -watt amplifier thecost is very much less.
40
-raVISIONJANUARY, 1934
EVERY OWNER of a RADIO SETSHOULD READ "AMATEUR
WIRELESS"
On Sale every WednesdayPrice 3d.
Add to the enjoyment of radio by having a clearunderstanding of how your set works.To have a fundamental knowledge of wireless meansthat should any trouble develop, or improvementsor additions be required, you can do them yourselfwithout having to resort to the trouble and expenseof professional assistance.AMATEUR WIRELESS will help you to obtain thisknowledge for the reason that all technical matters,illustrations, and diagrams are explained in simple,everyday language, to be understood by everybody.
41
'
ANSWE ix?0 0 0Cathode Rays :Scanning Lines
I have read the articles on thecathode-ray tube with interest asit seems to be the solution of theproblem of receiving a largenumber of lines. How far can onego in the number of lines pro-duced ?
Theoretically, of course, there is nolimit to the number of lines producedby the time -base since its speed oftraverse can be increased to 5,000 persecond easily. In practice the limit isset by the size of the scanning spot andthe dimensions of the picture. In thepresent tubes the limit would appear tobe about 18o lines, but i zo lines can beproduced with ease.
The Sound of theTransmissions
I have often tuned in the tele-vision transmissions and listenedto the sounds produced. My in-tention now is to make a simpledisc receiver, and I should like toknow whether, when receiving apicture, the noise of the broadcastis heard .-T. G. (Newcastle -on -Tyne).
No, the noise is not heard, as thesignals are fed to the neon lamp insteadof the loudspeaker, and this lamp con-verts the varying signal impulses intothe light and shade of the televisionpicture. A separate receiver is, ofcourse, used for receiving the soundsignals which accompany the pictures.The sound is broadcast from MidlandRegional and the vision from LondonNational.
Range ofTelevision Reception
My home is in Edinburgh, andthough I am anxious to take uptelevision it seems to me that thedistance from the transmitter istoo great to get any results. Willyou please advise ?-F. E. R.Edinburgh).
There are many amateurs in Scotlandwho are receiving television success-fully, both with disc and mirror -drumequipment. Naturally conditions area little more exacting, but if you canreceive the London National station at
fair loudspeaker strength, you will beable to receive the pictures. Reportsof excellent reception have been re-ceived from Madeira, and several ama-teurs in the South of England receivethe French and German transmissionsquite successfully.
Cathode Rays andWeak Reception
I am troubled with fading of theLondon National and the signal isnot very strong. From what Ihave read, the cathode-ray tuberequires very little voltage tooperate it. Would it be betterthan the mirror -drum for mypurpose ?
Yes. Provided that your outputstage will give good headphonestrength it should be sufficient to modu-late the C.R. tube. The fading ofwhich you complain will have to becounteracted by some form of A.V.C.or you will find it difficult to keep thepicture in synchronism.
Enlarging the 'Image
I am using a lens to enlarge theimage produced by my disc re-ceiver, but I find that the pictureis distorted when the lens is placedin the position to give the largestpicture. Do you consider that Iam using an unsuitable type oflens, and can you suggest amethod of obtaining high magnifi-cation without distortion ?-R. D.(Leicester).
ANSWERS TO QUERIESAn expert service is available to assist
readers who experience difficulties in .theconstruction, operation and maintenanceof television apparatus or associated wire-less receivers and amplifiers.
The following rules should be ob-served :
Please write clearly giving all essentialparticulars.
A stamped, addressed envelope andalso the coupon on the last page mustaccompany all queries. Not more thantwo questions should be sent at any time.
Reply will be made by post, usuallywithin twenty-four hours.
Queries should be addressed to theQuery Department, TELEVISION, 5 8-61 ,Fetter Lane, London, E.C.4.
JANUARY, 1934
EMTThe lens which you are using is evi-
dently of short focus, and though thiswill give high magnification it is alsobound to produce distortion. Highmagnification without an undue amountof distortion can only be obtained byusing a combination of two lenses.Focal lengths of 9 and 18 inches aresuitable values, and the distance be-tween them should be from ti to zinches. The focal length may be de-termined by producing a spot of lightfrom the sun and measuring the 'dis-tance between the spot and the lens.Lenses specially made for the purposeare obtainable from advertisers.
The Outputto the Neon
Is it quite essential that a 1-1transformer be used on the outputfrom the receiver to couple theneon ?-P. A. (Ely).
Transformer coupling for the outputis only one method employed. Detailsof several other schemes are shownpictorially on another page.
Operating aKerr Cell
I have made up a Kerr cell, butI can only get a small amount oflight to pass through it when thevoltage (which is approximately370) is applied to the plates. Canyou give me some indication whatis likely to be at fault ?-B. D.(Hendon).
If the electrical connections to theplates of the cell electrode assembly areall right, the fault is probably eitherthat the light rays do not pass, due tothe electrodes not being in line, or thatthe nicol prisms are not correctly set.Test the line-up by removing thesecond Nicol (analyser) and adjustingthe plates, etc., until the light raysemerge. Then replace the Nicol androtate it so that all emergent light isblacked out. The voltage applied tothe plates should then produce aneffect. For maximum effect the Nicolshave to be set correctly. With theoblique -ended prisms, the ends arediamond section and the polariser isfixed with the diamond section at anangle of 45 degrees to the plane of theelectrodes of the cell.
4z
JANUARY, 1934 li-AgYJS41011
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In order that anyone you know whois interested in television but is notyet a reader of the only publicationentirely devoted to this subject-"TELEVISION," a complimentarycopy will be sent to him gratis andpost free, if you will kindly sendyour request on a postcard, givingboth your own and you rfriend's addressand attach the special coupon below.
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JANUARY, '934
Apparatus for the ExperimenterTHE EDIS WAN C.R. TUBE :: LOW-PRICED POLARISERSAND ANALYSERS :: THE PHILIPS' PROJECTION LAMP
The EdiswanCathode-ray Tube
WE have received from the Edi-swan Co. one of their new type" T " cathode-ray tubes, which
is primarily intended for use in tele-vision viewing, but which can also beused in radio research. The illustra-tion shows the main details of the tube,the operation of which is no doubtfamiliar to our readers.
The electrode structure is assembledon the usual glass " pinch," the anode
The EdiswanCat bode -raytube for televi-sion. The baseis a standard4 -pin which al-lows of a valveholder beingused for theconnections.
lead being shielded by a glass tubeprojecting from the pinch to avoid thepossibility of leakage at high voltages.Passing through the flat anode andinsulated from it by mica washers arethe supports for the deflecting plates.Above the plates themselves is a metalring which serves as a collector forstray electrons returning from thescreen, and prevents excessive deflectorplate current.
The overall dimensions of the tubeare 46 cms. long by 14 cms. diameterat the screen end. The basing arrange-ments are simple and make for ease inconnecting the tube in circuit. Theelectrode structure is connected to the4 pins of a standard 4 -pin base, andabove this base surrounding it is anebonite collar on which 4 terminalsare mounted making connection to the4 deflector plates. These are labelledA', A2, B", B2 respectively.
The rating of the tube is as follows :Filament current-I amp. approx.
at .45 volt.Anode voltage -30o- I ,500.
Each tube when sent out is labelledwith the correct operating current, andthe manufacturers state that this shouldnot be exceeded except in certain cases,when a tolerance of 5 per cent. isallowed.
On connecting the tube to the excitercircuit recommended in the leaflet sup-plied, a vivid green spot appeared onthe screen which reduced to .75 mm.diameter under the correct focusingconditions. It should be noted that itis usually necessary to re -focus whenthe spot is expanded to a line, and aline can easily be obtained smaller inthickness than the spot itself.
For wave -form observations, it wasfound possible to operate the tubesatisfactorily with an anode voltage ofonly 450 without taking special precau-tions to screen the end of the tube fromdirect light. For television, however,it would appear advisable to increasethe anode voltage to a minimum of 900.
The sensitivity of the tube (i.e.,deflection in mms. per volt applied tothe deflector plates) varies inversely asthe anode voltage of the tube, andtherefore no definite figure can begiven for this. From tests made at450 volts, the sensitivity is approxi-mately .7 mm. per volt.
The Ediswan Co. have been experi-menting for some time with simplecircuits for the application of this tubeto the reception of television images,and they state that they will be pleasedto send information to any interestedenquirer. We have witnessed a de-monstration of the tube in the reception
of Baird television, and the results areextraordinarily good considering thesmall size of screen (approximately14 cms. by 6 cms.).
The colour of the screen is in markedcontrast to the reddish glow of a neon,and, if anything, is more soothing tothe eye. A slight trace of persistenceof the glow of the fluorescent materialassists in toning down the harshness ofthe picture and increases the steadiness.
An interesting point is the negligibleenergy required to produce the picture.The modulating voltage is applied tothe shield of the tube, and a totalfluctuation of some o-zo volts is allthat is required to produce full modula-tion in intensity of the beam. This isan advantage for users of battery setswho cannot provide heavy poweroutputs.
The list price of the tube is £6 6s.,and, considering the multitudinous usesto which it can be put, it represents agood investment. The standardscreen supplied is green, but a blue -white screen adapted for photographicwork can be obtained. The manufac-turers are The Edison Swan ElectricCo., Ltd., Radio Division, 155, CharingCross Road, London, W.C.z.
Low -PricedPolarisers and Analysers
Messrs. Wilburn & Co., of Car-melite Street, London, E.C.4, havesubmitted for test a pair of Nicol sub-stitutes. The Nicol substitute is, ofcourse, well known. It consists of a
GLASS SLIDES AT 35° BRASS TUBE
SEALING COMPOSITION
number of thin glass plates placed atan angle of about 35 degrees within a
44
JANUARY, 1934 ;.'1, r.Y1S1
Miscellaneous AdvertisementsReaders who wish to sell, exchange
or purchase apparatus will find thiscolumn a very successful means ofdisposing of their surplus gear, orobtaining new apparatus at bargainprices.
SEND your orders and enquiries for all televisionapparatus to us. We supply all " Mervyn " and Bairdcomponents, including motors and synchronisers,scanning discs, neons, mirror drums, lenses, resistances,Kerr cells, nicols, etc. Motor, universal type, i in.spindle, with ball bearings, price 30/-. New craterpoint lamp to carry 3o m./a., i2/ro/-. Unisphere drumkit, £3/ io/- and new mirror drum, £2121- for home con-struction. Synchronising transformer, t3/- post.Synchronising gear, 22/6 complete. Handbook "EasyLessons in Television," 2/- post free.-H. E. Sanders &Co., 4, Grays Inn Road, London, W.C.x.--TelephoneChancery 8778.
THE proprietor of British Patent No. 394824 re:atingto device for exploring or reconstituting an object orimage for use in systems of picture telegraphy, copyingtelegraphs or television or the like (including an im-proved means of constructing a mirror drum) isdesirous of entering into negotiations with interestedparties with a view to the exploitati. n of the inventioneither by the sale of the patent rights or by the grantingof licences on reasonable terms. All enquiries shouldbe addressed to E. Wotton, 28, Cavendish Street,Ramsgate.
A. DOSSETT, commercial artist and draughtsmanfor all technical diagrams, illustrations and layouts.-High Holborn House, 52, High Holborn, W.C.2.Holborn 8638.
TELEVISION-All components supplied by JohnSalter, member Television Society, z3, FeatherstoneBuildings, High Holborn, Pioneer maker since1927 of television apparatus. Stroboscopic scanningdiscs, motors, lenses, and all parts. Lists free.
A. MATHISEN, B.Sc., patent agent specialist inobtaining patents for television and radio inventions.Working drawings, circuit diagrams prepared for sub-mission to manufacturers. Exploitation advice.Preliminary interview free.-First Avenue House, HighHolborn, London, W.C.x. Holborn 895o.
ELECTRIC MOTORS used double -ended, fullyguaranteed, x9/9. Wide range new motors. Slideresistance, 7/6 ; speed indicator, ; scanning disc,12/6 ; beehive neon, 3/8 ; synchroniser tooth wheels,3/6 ; best, 4/9 ; poles, 4/- ; pole holders, 41- ; coils,5/1i per pair; yoke, 3/6. Disc receiver constructor,illustrated, 1/6. Mirror receiver constructor, 1/1 x.Mirror drums, nicols, lenses, etc., in illustrated listsfree, notes rd.-Ancel Gine Television Co., 26, High -bury Terrace, Nr. Highbury Station.
TELEVISION RECEIVER. Baird disc modelcomprising motor, synchroniser, disc, lens system,neon, speed control and moving coil speaker mountedin polished wooden cabinet. Practically new, £6/ xoi-.Mains operated power amplifier, output stage, twoPP.5 goo valves in push-pull, complete with valves, £5.B.T.H. universal type gramophone motor mounted insmall table £2.- Hanwyn, ' St. James Gardens,Westcliff, Essex.
TELEVISION WITHIN THE REACH OF ALL.Disc Receivers, Amplifiers and Kits, from 30/- each." Television " and " Amateur Wireless " guaranteedfirst specified kits complete. Lowest prices. Lists free.-" Melforad," 5, Queen's Place, Hove (Trade sup-plied).
MIRROR SCREW-We can supply a kit of partscomplete, £2/xo/-. This can be seen and technicaldetails explained. We have for disposal the originalmirror drum receiver described in this journal by E. I..Gardiner, B.Sc., and exhibited at the Radio Exhibition.Enquiries invited. Apparatus can be sent C.O.D.Send for illustrated list.-H. E. Sanders, 4, Grays InnRoad, London, W.C.x.-Telephone Chancery 8778.
ORIGINAL Pressler Cathode Ray Tubes, Cells,Crater Lamps, as reviewed in " Television," are mail-able from the sole British representative EUGEN J.FORBAT, 28-29, Southampton Street, Strand, W.C.2.Temple Bar 86o8.
mount, but this, we believe, is the firsttime that this article has been madecommercially for television purposes.The construction will be clear form theillustration.
Specially selected glass plates areused in the Wilburn composite prisms,and these are cemented securely intothe brass mounts. Tests showed thatan entire blacking out could not beobtained when the prism substituteswere placed in the correct position,but that the efficiency compared withactual Nicols was approximately 8o percent. This slight advantage was com-pensated for to a considerable extentby the fact that the surface is largerthan with a normal Nicol, and thatdamage is not likely to result fromheat ; they may thus permit a greaterlight intensity to be used. They areeminently suitable for use with an arclamp. The price of the pair is 17s. 6d.,which is less than half the price of anequivalent pair of Nicols.
TheProjection Lamp
We have received from PhilipsLamps, Ltd., of 145, Charing CrossRoad, London, W.C.z, a projectionlamp suitable for use in conjunctionwith mirror -drum apparatus. Thelamp is rated at 1 oo watts and thecandle power is approximately 200.It is intended for 1 z -volt operation.The filament is of the bunched variety,so that the total length of the incan-descent portion is about - in., thusproviding a highly concentrated andintense source of light. The lamp hasthe usual standard screwed base whichwill fit any standard holder, butMessrs. Philips supply a special basemade of bakelite which is detachableby means of a quick -screw arrange-ment, and also is provided with anautomatic locking device to enablequick adjustment of the position ofthe lamp to be made.
"TELEVISION" COUPONFOR FREE INQUIRY SERVICE
Name
Ad dressJanuary, 1934.
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The charge for advertisements in thesecolumns is 12 words or less 2/-, and 2d. forevery additional word. All advertisementsmust be accompanied by remittance. Chequesand Posi al -orders should be made payable toBernard Jones Publications Ltd., and crossed,and should reach this office not later than the15th of the month previous to date of issue.
CALL AND INSPECTTHE LATEST TYPEOF PROJECTORCOMPONENTS, Etc.
H. E. SANDERS & CO.,4 GRAYS INN RD., LONDON, W.C.1.
Co., Ltd. .. .. Cover iiWhiteley Electric Co. .. 43Wilburn & Co. .. Cover ii
'1;-1.;'.11S410d JANUARY, 1934
Useful Gadgets to Add
our Set
J
CONTENTS of WIRELESS MAGAZINE, JANUARYFOR THE CONSTRUCTOR
THE 1934 A.C. QUA DRADYNE. By the" W.M." Technical Staff ..RESULTS OF AN EVENING'S TEST
THE A.C. TRANSPORTABLE. Designed bythe " W.M." Technical Staff ..
MORE ABOUT THE MERRYMAKER SUPER ..THE ALL -WAVE THREE. By the " W.M."
Technical Staff ..A TEST OF THE ALL -WAVE THREEUSING YOUR SET ON THE SHORT WAVES.
By Kenneth Jowers
TECHNICAL FEATURESMEASURING THE HEAVISIDE LAYER. By
G. S. Scott ..Flow MANY VALVES IN YOUR 1934 SUPER ?
By S. Rutherford WilkinsMAKING THE MOST OF THE SUPER -HET. By
J. H. Reyner, B.Sc., A.M.I.F..E.WE TEST BEFORE You Buy. By the "W.M."
Set Selection BureauATLAS A.C. THREE-VALVER, MODEL 334
COSSOR A.C. SUPER -HET. MODEL 635 ..SUNBEAM UNIVERSAL A.C.-D.C. MIDGET
KOLSTER BRANDES 666 A.C. SUPER -HET.EKCO MODEL 74 BATTERY SUPER -HET, ..TESTS OF NEW APPARATUS ..OPERATING THE CATHODE-RAY TELEVISION
RECEIVER. By J. H. Reyner, B.Sc.,
LIGHT MODULATION IN TELEVISION. ByHarold Corbishley .
INTERMEDIATE FILM TELEN'Is1ON. By G.Arthur
GENERAL ARTICLESG ui nE TO THE WORLD'S BROADCASTERS. By
Jay CooteWoELD'S BROADCAST WAVELENGTHSI s NE WITH THE TRADE. By FetterDE -BUNKING RADIO. By Percy W. Harris,
31.Inst.Rad.E.THINGS TO COME. By Morton BarrUSEFUL GADGETS TO ADD TO YOUR SET .." RADIO MUSIC IS ALIVE." By Watson
A ROOM FIT FOR YOUR SETMUSICAL TOUR OF THE ETHER. By Whitaker
WilsonREAL RESEARCH ..RADIO RIGA .,ON THE HIGH SEAS. By Derek EnglandRADIO KOOTWIJKRADIO IN YOUR CAR. By Kenneth JowersRADIO MEDLEY. By BM -PRESSMUSIC OF THE MONTH. By T. F. Henn ..B.B.C. ON LIGHT ENTERTAINMENTYOUR DANCE MUSIC COME FROM-ON
..THE CREST OF THE WAVES. By Jay Coote
GRAMOPHONE FEATURESTURNTABLE STROBOSCOPES. By P. Wilson,
CHOOSING YOUR RECORDS. By Whitaker -Wilson
ADDITIONAL RECORDS. Reviewed by Chop-stick
Since the first days of radio, the word " gadget " hasalways played a prominent part. No matter howgood your present set may be, there is always asimple way of improving reception by the addition ofsome gadget or another. This interesting article inthe January Wireless Magazine describes many usefulaccessories and their advantages are fully explained.
For the constructor there are three splendid sets tobuild. The A.C. Quadradyne, a 4-valver ; the A.C.Transportable, a self-contained 3-valver ; and thebattery All -Wave 3, which covers two short-wave bandsin addition to the normal medium and long waves.
This January issue contains a host of useful andentertaining features that no set owner should miss.
GET YOUR COPYof the JAN. ISSUETO -DAY, Price 1 /-
WIRELESS MAGAZINE
Muted for the Proprietors and Publishers, BERNARD JONES PUBLICATIONG, LTD., 58-61 better Lune, London, E.C.4, by THE Bios AT COOMBELANDE, LTD.,Andlestone, Surrey. Sole Agents for South Africa : CENTRAI. NEWS AGENCY, LTD. Sole Agents for Australia and New Zealand GORDON AND GOTCII,AUSTRALASIA). LTD.
