ELECTRONIC VOLTAGE REGULATOR

equations of Schuster. 6 They are true only inthe case that d is constant with x, or in otherwords, that the diffuse radiation is diffusethroughout the medium.

THE CASE OF THE ATMOSPHERE

In the case of the terrestrial atmosphere belowthe ozone region is approximately zero for thewave-lengths of the visible spectrum. Duringdaylight io is the light of the sun that falls onthe "top" of the atmosphere, and a is approxi-mately zero. If t is the optical thickness of theatmosphere, a is related to the average bright-ness of the daylight sky to an observer lookingupward from a point at an optical distancex below the top, and at to the average sky bright-ness seen by an observer on the surface of theearth. b is related to the average brightness ofthe atmosphere seen by an observer above theatmosphere looking downward.

With 3 =d=O, Eqs. (13) to (17) reduce,respectively, to

a = Io(l +gt)-l { (1 +gt) (C- CX)-gx(C-CT+T)}, (22)

at = I(1 +gt)-1(C-CT-gtT), (23)

b = Io(l +gt)-' { (1 +gt) (C- CX+X).- (gx+1-r)(C-CT+T)}, (24)

bo= I(1 +gt)-tX { (1 +gt)-(1-r) (C-CT+ T) }, (25)

bt=Ior(1+gt)'(C- CT+T), (26)6 A. Schuster and J. W. Nicholson, The Theory of Optics

(Longmans, Green and Company, 1924), third edition,page 271.

JANUARY, 1943

whereIo=io cos ',

g= (-r) -Vd) d,C = 7C+ (-ad) (d/>) COS go (27)

X = exp (-a, sec x),T= exp (-a, sec *).

Equations (22) to (27) comprise part of adetailed theory of the brightness and polarizationof all points of the daylight sky viewed by anobserver at various altitudes above the earth.The detailed theory and comparison with ob-servations will be given in a future paper.

THE CASE OF THE DEEP SEA

In the case of the sea io and ao are the light ofthe sun and the sky, respectively, that enter thesurface of the sea. For light of the visible spec-trum both absorption and scattering are im-portant, and Eqs. (13) to (17) apply directly toa sea of finite depth and take into account thereflectivity of the bottom. For an infinitely deepsea, that is, a sea so deep that the amount oflightpenetrating to the lower levels is negligible,t= o and at=bt=O.

With t= o, Eqs. (13), (15), and (16) reduce,respectively, to

a =aoe-zr+IoC(e-,-X),b = boe-rx+IoD(e-zxX),

(28)

(29)

bo= [ao(t - -qd)6yd+i(l - C)oc- IoD(y-A)]X [fd+ (1- ?d)d+'Y]_'. (30)

The application of these equations to opticalmeasurements of the sea will be given in a futurepaper.

J. 0. S. A. VOLUME 33

Electronic Voltage Regulator for Precise Densitometric Measurements*

H. W. DIETERT AND M. F. HASLERHarry W. Dietert Company, Detroit, Michigan

IT is a well-known fact that the precision ofdensitometric measurements, and thus the

precision of quantitative spectrochemical deter-minations, is dependent upon the constancy of

* Presented at the Tenth Summer Conference on Spec-troscopy and its Applications held at the MassachusettsInstitute of Technology, Cambridge, Massachusetts, July20-22, 1942.

illumination of the densitometer lamp. Theproblem of maintaining this constancy proves tobe a very difficult one, as to maintain the lightoutput accurate to 4±0.2 percent demands volt-age control accurate to about ±t0.07 percent.

In reading the advertising literature on voltageregulators, this does not seem particularly difli-

45

H. V. DIErERT AND I. F. HASLER

1/O AC

FIG. 1. Circuit diagran

cult however, as there are on the market severalsimple regulators of the resonance transformertype, or the saturated core type, which claim tolimit the output variation to one volt for aninput variation from 90 volts to 130 volts. Thisindicates a degree of correction of at least twentyto forty depending upon the shape of the outputvs. input curve. If we assume a factor of thirty,this limits a z43-volt change at the input to4L0.1 percent at the output. This would bealmost good enough for accurate densitometricmeasurements for this degree of input changeand would certainly suffice when ordinarily uni-form line voltage was available.

However, the difficulty with this type of regu-lator proves not to be related to the obviousthing, change of input voltage, but rather to aless well-known thing, change of input frequency.For instance, one well-known make of regulatorwill vary its output voltage by 1.6 volts for a 1percent change in input frequency. The questionthen arises what variation in frequency can beexpected in the usual commercial power installa-tion. In general, the newer power installationsare giving considerable attention to frequencystabilization. Probably the best one in this re-spect is the new installation at Boulder Dam, inwhich the frequency is never allowed to drift bymore than 0.1 cycle in 60 cycles. However, in the

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small power plants supplying some municipali-ties, the drift in frequency may amount to asmuch as 0.5 cycle in 60 cycles. On the average,of course, the output is made exactly 60 cycles,so that such power can be used to operate clocksand other synchronous devices. Thus, merelybecause a power company advertises that electricclocks may be used is no assurance that fre-quency variations do not occur. With these factsin mind, what voltage variations can then beexpected? In cases of excellent frequency stabil-ization, 0.1 cycle in 60, this amounts to a varia-tion of 0.26 percent in output voltage which,transposed into transmission readings, can causevariations of almost 0.8 percent. In cases of a0.5 cycle variation, fluctuation of 5 percent canbe expected in transmission readings on thedensitometer.

This state of affairs has led many people usingand building densitometers to the conclusionthat something as stable as a large capacitystorage battery floating across a charging line isthe only really good solution to this problem.However, anyone who has used an instrumentpowered by alternating current, which hasworked reasonably well, is very reluctant toaccept this solution and will only do so as alast resort.

From the above discussion it seems that the0

46

ELECTRONIC VOLTAGE REGULATOR

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output voltage experienced from four different regulatorsduring the wvarm-up period.

voltage in varying amounts so that the finaloutput remains constant at the desired value.Such a device furnishes alternating current inthe output, which is very desirable, as the sameregulator that furnishes constant voltage to thedensitometer lamp may then be used to powerthe densitometer amplifier, if such is required.For most other commercial uses to which aregulator may be applied, an alternating currentoutput is, of course, almost a necessity.

The arrangementfinally adopted is shown inthe circuit diagram, Fig. 1. Transformer 3951raises the input voltage from 115 v to about200 v, while transformer 3979 drops the outputvoltage back to 110 v. The two amplifier tubesacross the output of this transformer act as avariable resistance load, and thus by controllingthe grids of these tubes in a compensating manner,exact control of the voltage may be obtained. Byoperating these tubes at the correct grid bias,almost a linear change of current with change ofvoltage is obtained, so that they act very muchas a pure resistance. In this manner very littledistortion of wave form is introduced. Gaseoustubes, such as thyratrons, would, of course, givea large distortion of wave form.

The method of controlling the grids of theseamplifier tubes proves particularly suitable to theproblem in hand. Lights with fine filaments, andhence with low thermal inertia, are used as thecontrolling elements. In this way an integratedcorrection, independent of exact wave form, fre-quency, or voltage, is obtained, which tends tokeep the densitometer lamp very constant. Thecorrection has been applied in two ways, first,as a compensating effect, and second, as a regu-lating effect. By adjusting the position and in-

tensity of the lamp connected to the inputvoltage, compensation of almost all variationscan be achieved by means of the photo-cells andthe vacuum tube amplifier feeding into the gridsof the amplifier. However, use of just this controlproves not to be sufficient, as it suffers from theusual limitation attendant in all compensatingcircuits, namely, perfect compensation at onlyone input voltage. Thus, a second lamp is used,operating from the output voltage, which has atrue regulating effect-the ability to reduce anychange to a small fraction of itself. This lampalone will reduce a 10volt change to about 1volt, and when used in conjunction with thecompensating lamp, gives a very stable outputvoltage.

Just how stable this can be is shown in Fig. 2where the output of a linear densitometer isgiven as a function of input voltage. These re-sults appear very acceptable and show that for a

L5-volt change, the transmission readings willvary by about =t0.2 percent. Results over aperiod of ten minutes, shown in Fig. 3 indicatethe degree of constancy that can be expected inregular operation. Surges, one of which is shownin the last mentioned figure, are noticeable ifthey are large enough, but thanks to the lowthermal inertia of the controlling lamp and thehigh thermal inertia of the densitometer lamp,they do not introduce appreciable errors.

Besides the ability to compensate at any onetime, a regulator must also be able to maintainits output constant over reasonably long periodsof time-ten minutes to a half hour-while asingle plate or film is read in its entirety. Thispresented quite a problem, due to the large powerrequirements of the ARL-Dietert comparator-

FIG. 4. Change of

48

INORGANIC COMPOUNDS IN SPECTROGRAPHIC STANDARDS

densitometer for which the unit was designed. Ithad to be able to supply 160 watts of extremelywell-regulated power, which necessitated an in-put of between 400 and, 500 watts, and a dissipa-tion of almost 300 watts. The dissipation of thismuch power causes a considerable rise in tem-perature of the component parts of the regulator,which, in turn, causes an initial drift in outputvoltage until approximate thermal equilibrium isreached. This takes from fifteen to thirty min-utes, after which the regulator maintains itsoutput voltage very accurately over long periods.Figure 4 shows this initial drift and subsequentstabilization for four different regulators. Thedifferences between units can doubtlessly be

JANUARY, 1943

attributed to different temperature coefficientsfor the different component parts. The part mostsensitive to temperature changes proved to bethe 20-meg. resistor in the photo-tube circuit.Experiments extending over many months indi-cated that a slow, continued change could beexperienced with certain makes of resistors overmany hours. This, of course, caused undesirabledrifting. However, two 10-meg. G.H. resistorsproved most successful in avoiding this effect.Since employing these resistors, very stableoperation is experienced after the warm-upperiod. This period has been reduced in time in thenew models by ventilating the whole unit moreeffectively than was done in the older models.

J. 0. S. A. VOLUME 33

Preparation of Purified Inorganic Compounds for Use in Spectrographic Standards*

RAY C. HUGHEStUniversity of Florida Agricultural Experiment Station, Gainesville, Florida

THE lack of sufficiently pure chemicals forT use in the preparation of standard samplesrequired for comparison purposes, or for theestablishment of working curves, often deter-mines the lower limit at which spectrographicanalyses may be made with the desired accuracy.Available chemicals, even those of the highestpurity obtainable, may be unsatisfactory for cer-tain purposes. The need for materials of highpurity for use in the analysis of metals and alloys.has been recognized by the American Society forTesting Materials,", 2 and a survey of sources of"super-purity" metals has been made.' A similarneed exists for compounds of high purity for usein preparing standards for spectrographic analysis

* Presented at the Nioth Summer Conference onSpectroscopy and its Applications at Massachusetts Insti-tute of Technology, July, 1941, and published with thepermission of the Director of the Florida AgriculturalExperiment Station.

t Research Assistant, Soils Department, 1939-1941. Pres-ent address: Test Laboratory, United States Navy Yard,Philadelphia, Pennsylvania.

1 T. A. Wright, "Report of subcommittee V on standardsand pure materials," Proc. A.S.T.M. 37, 531-7 (1937).

2 T. A. Wright, "Superpurity metals," Proc. A.S.T.M.37, 538-42 (1937).

3 T. A. Wright, "The Availability of high purity metalsfor spectroscopic purposes," Spectroscopy in Science andIndustry (John Wiley & Sons, Inc., New York, 1938), pp.47-50.

of agricultural samples. Since satisfactory chem-icals are not obtainable, it has been necessary toprepare materials suitable for the proposed useby removal of objectionable impurities from theavailable chemicals. The need for purified com-pounds for spectrographic use, and the obviousapplications of purified chemicals to other prob-lems, was considered to justify a description ofsome methods which have been found useful forthe preparation of compounds of the requiredpurity.

Published information on methods for thepreparation of purified chemicals is widely scat-tered and difficult to assemble, since most of ithas been published as a part of researches inwhich the preparation of purified compounds wasnot the primary object; consequently, these pub-lications have often been abstracted and indexedwith little regard for purification problems.Archibald4 has collected and made available mostof the older work, with emphasis on the prepara-tion of samples for atomic weight determinations.

It should be pointed out here that the require-ments for atomic weight samples and for spectro-

4 E. H. Archibald, The Preparation of Pure InorganicSubstances (John Wiley & Sons, Inc., New York, 1932).

49