Physics. - Methads and apparatus used in the cryagenic laboratary. XXII. A cryastat far temperatures belaw 1 ° K . By W. H. KEESOM. (Communication N0. 195c from the Physical Laboratory at Leiden.)

(Communicated at the meeting of May 25, 1929).

§ 1. lntraductian. 13 Oct. 1921 KAMERLINOH ONNES 1) succeeded in diminishing the pressure above liquid helium to 0.013 mmo mercury. To be su re KAMERLINOH ONNES in the mentioned publication rounded oH the pressure obtained upwards to 0.02 mmo The corresponding temperature he calculated to be 0.82° K, being the lowest tempera tu re that was reached till then 2). This temperature, which is still the lowest that has been reached, was obtained in the small Oewar-glass e, see Fig. 4, loc. cit., when this was about half WIed. The quantity of liquid helium was probably about 1 cm ::!

The construction of the high vacuum pump with great capacity::!) described in Comm. N0. 195a suggested progress in two directions.

lst. It may be possible to diminish, still more than was possible then. the pressure above a small quantity of liquid helium, which is protected, in the way as described in Comm. N0. 159, as accurately as possible against heat entering from outside, and so to reach still lower temperatures.

2nd • It may be possible to cool a larger quantity of liquid helium to near the temperature reached already by KAMERLINOH ONNES. It will not be necessary then to take all those measures against heat transfer, which were necessary when KAMERLINOH ONNES made his experiment. So it will be possible to bring within the space cooled to those low temperatures measuring apparatus, which are connected to the space without f.i. by glass capillaries or conducting wires.

Our op in ion was that to work out this last problem was the first thing needed for the progress of scientific research.

Hitherto it has been possible at Leiden to make experiments in liquid helium boiling under a pressure of about 0.5 mm o The lowest pres su re at which a measurement has been made, was 0.46 mmo 4). Not only for meas­urements of the vapour pressure curve of helium for fixing the temperature scale of the lowest temperatures but for measurements in connection with

1) H. KAMERLINGH ONNES, Trans. Faraday Soc. Vol. t8, Comrn. Leiden NO. 159. 2) According to the formula, which VERSCHAFFELT, Comm. Leiden Suppl. NO. 49,

p. 26, calculated for the vapour-pressure of helium, to p = 0.020 mrn corresponds T = 0.84 and to p = 0.013. T = 0.81.

3) W . GAEDE and W. H. KEESOM. These Proceedings 31, 985, 1928. Comm. Leiden NO. 195a.

i) W. J. DE HAAS and J. VOOGD, These Proceedings 32. 214. 1929. Comm. Leiden NO. 193b. According to the formu1a of VERSCHAFFELT mentioned note 2, to th is pressure a temperature of 1.160 K. corresponds.

711

superconductivity, it was urgently necessary to extend downwards thc temperature range in which measurements may be done.

§ 2. The high vacuum pump instaUation and the cryostat lor temperatures below 10 K. As was communicated in Comm. N0. 195a th~ cryogenic laboratory possesses two high vacuum pumps with great ci'lpacity. The capacity of the first (PI' Fig. I) amounts to 422, that of th!.'

I i o IQ .0 30 40 SC

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t Fig. 1.

second (P"2) to 270 L. He/sec. It was convenient to put these two pumps in parallel, as Fig. I shows. The two have an exhaust capacity, measured at F I' of 675 Liter helium per second.

V is a tube, which conducts to the Burckhardt-pump, which produces the preliminary vacuum. This preliminary vacuum is measured with the Mc Leod gauge Me,. Both pumps PJ and P2 were now inwardly cooled by a current of alcohol. which was cooled by liquid air to about -ISO C. This occurred in the refrigerator R insulated with kapoc (Fig. 2). The liquid air enters through Rt3 in the vessel Rb t with insulated bot tom (in order to prevent too intense cooling of the alcohol there), the air evaporates, and passes through spiral Rsp into the open air. The refrigerated alcohol Ra is driven by the pump Rp through the tubes Rl l and Rl:!. to the cooling spirals (Sp in Fig. lof Comm. N0. 195a) of the high vacuum pumps PI and P 2

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(Fig. 1) and back again to R. The temperatures of the alcohol are read on

the thermometers Rth t • Rth2 and Rth3 •

T (Fig. 1) is the common exhaust tube of the two pumps. It has a diameter of 30 cm. Al and A 2 are two cylindrical tubes of iron fastened by insulated Germansilver tubes to the exhaust tube : they are kept Wied with liquid air to condense the mercury vapour which may come from thc pumps into the exhaust tube. Level indicators show the level of the liquid "ir in these tubes. In order to keep the mercury vapour. that might paS3

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fig . 2.

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713

these tubes, still as much as possible from the cryostat, a copper gauze G is provided, which is connected wüh the liquid air~coolers Al and A:! by a l:eat conducting plate PI.

A funnel shaped part f is soldered to the exhaust tube and ends in a flat box B with semicircular border. In the bottom and the cover of this box circular holes are provided. The cryostat glass C (Fig. 3) is fastened intI) the bottom in the usual way, in the hole above a lid Co is placed, in which several holes have been bored to fa sten the measuring instruments (thermometer capillaries, etc.).

Fig. 3 is a more detailed figure of the cryostat. It is fi1led with liquid helium from the cryostats connected to the helium liquefactor by the vacuum si ph on tubes 5 2 and 51. When the cryostat glass is filled, the valve Va is closed, the siphon 5 2 is disconnected, af ter which the free end of 51 is completely closed with rubber, in order to prevent the possibility of leakage when reducing the pressure in the cryostat.

The contents of the cryostat glass (inner diameter 50 mm., length 55 cm.) are shown in Fig. 3 and are the same as those used for the measurement:3 of Messrs. WEBER and SCHMIDT, and myself, on May 24th (see § 3), on the vapour pressure curve of helium for the lowest temperatures. Th l and Th 2 are the reservoirs of two helium thermometers, Tha is the tube of the auxiliary thermometer, which serves to determine the correction for the emergent stem of the main thermometers. V p is the reservoir of a vapour pres su re apparatus, St is the mechanica I valve stirrer, Tu is an open tube reaching into the Iiquid. Mal> Ma2' Ma3' Ma4 are heated wire manometers to measure the pressures in Th! , Th:!, Tha and Vp.

The pressures in the vapour pressure apparatus V p, and in the open tube Tu could moreover be measured with la mercury manometer Hg or with the Mc Leod gauge Me2 (Fig. I), with which also the pressure in thc exhaust funnel at P l can be measured.

§ 3. Experiments . A first experiment with the instaHation described in § 2 occurred on May 3 rd . When several quantihes of liquid helium had been transferred to the cryostat glass the siphon 5 2 became defective. Nevertheless after reducing the pressure, first with the Burckhardt­pump, and then with the high vacuum pumps, the cryostat glass was stiH filled to a height of 10 cm. This was at 17h .43. The pressun;, measured in the exhaust funnel, amounted to 0.0074 mmo However the height of thc helium was not sufficient to cover the thermometer reservoirs, so that the planned thermometer measurements in connection with the vapour pressure curve could not be made that day.

We pumped farther to see, whether the temperature would still diminish. At 18h 25 the pressure, measured in the exhaust funnel, was 0.0068 mmo The height of the helium in the cryostat glass was 6 cm. The evaporation of the helium amounted to about 22 L. gas per hour measured at normal pressure and temperature. The experiment was now finished .

46 Proceedings Royal Acad. Amsterdam. Vol. XXXII. 1929.

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May 15th the experiment was repeated. In the meanwhile the siphon 5 2

had been improved. Measurements with the helium thermometer, at pressures m the eryostat of 0.5 to 0.06 mmo we re made. The eryostat eontained about 230 em3 liquid helium, while the apparatus immersed in the liquid had iJ

volume of about 60 em3 • So a space of about 300 cm3 • was cooled to thes~ low temperatures.

A new experiment .to continue the measurements concerning the vapour pressure curve took place on May 24th . Then measurements were again made to a pressure of 0.06 mmo

§ 4. The tempera tu re reached. The results of the direct temperature measurements with the helium thermometers will not be known for some time, when the thermomolecular pressure difference between thermometer bulb and manometer has been submitted to a further research. We will provisionally satisfy ourselves with an estimate.

Starting from the measurement of the pressure in the exhaust funnei mentioned in § 3 : 0.0068 mm., we put the pressure above the liquid surface 0.03 mm., from an estimate of the viseosity in the eryostat glass. This estimate was eheeked at a little higher pressure by observing the differenee of pressure between a tube I 1.5 mm wide, dipping in the liquid, and the exhaust funnel. While awaiting the results of the above mentioned measurements of the vapour pressure curve, we may remark that aeeordinq to the temperature seale given by VERSCHAFFEL T's formula 1) a tempera­ture of 0.87° K. should eorrespond to this pressure. An extrapolation based on provisional results of the measurements of the vapour pressure curve to 0.6 mmo would give 0.85° K.

So we may conclude, that it is now possible by the installation described in th is eommunieation to make measurements to a temperature of about 0.85° K. in a spaee of about 300 em3• By this means the temperature range in whieh measurements ean be made, is extended downwards by about 0.3 degree.

I am glad to render my eordial thanks to G. J. FUM, chief of the technica I staff of the eryogenic laboratory, for his intelligent and unwearied aid in the construction and the arrangement of th is installation.

Postscriptum.

Already on May 3Ist a first noteworthy result has been obtained with these applianees, when I had the pleasure of putting it at the disposal of Messrs. DE HAAS and VOOGD, who then found gallium to get super­eonduetive as had been anticipated 2).

I) See p. 710, note 2. 2) W. J. DE HAAS and J. VOOGD, loc. cito p. 710, note i.