1. Introduction. For several measurements, e.g. for measurements of the specific heat, it is desirabie, to have at one's disposal, even for the

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Physics. - Resistance thermometry at the temperatures of liquid helium. By W. H. KEESOM and J. N. VAN DEN ENDE. (Comm. N0. 203c from the Physical Laboratory at Leiden).

1 Physics. - Resistance thermometry at the temperatures of liquid helium. By W. H. KEESOM and J. N. VAN DEN ENDE. (Comm. N0. 203c from the Physical Laboratory at Leiden). (Communlcated at the meeting of November ). 1. ntroduction. For several measurements, e.g. for measurements of the specific heat, it is desirabe, to have at one's disposal, even for the temperature~range of liquid helium, a secondary thermometer, that indicates quickly. As such we can consider first of all the resistance thermometer. Previously, use has been made of the fact, that at the temperatures mentioned, the resistance of constantin and manganin changes sufficiently to be used for the thermometry of those temperatures 1). However a difficulty in using these wires was that the icepoint~resistance did not stay constant; af ter cooling it often appeared that a discontinuous change had taken place. Particularly it appeared that magnetic fields could cause disturbances. hus we found for a constantin wire, that its resistance undergoes a relatively large change under the influence of a magnetic field, and that this change does not entirely disappear on switching oh the field. herefore we have looked for other alloys, the resistance of which changes sufficiently in the temperature~range of liquid helium, so that they can serve as resistance thermometers, and on which magnetic fields have no disturbing influence. 2. We investigated the following alloys: platinum~silver, platinum~ 10 % iridium, brass, phosphor~bronze, furnished by HARMANN and BRAUN A. G. in 1928, and "technically pure" platinum, furnished by HERAEUS in he diameter of the wires was ca mmo hey were bifilarly wound round glass, fastened and insulated in a celluloid layer, protected by paper and provided with conducting wires of copper. he wires we re pre~cooled several times, but not annealed. he resistances were measured at the temperatures of melting ice, liquid air (for orientation), liquid hydrogen and liquid helium. 3. Results. he wires Pt~Ag, Pt~r and echn. Pt showed a continual change of the resistance with temperature. n the region of liquid 1) H. KAMERLNGH ONNES and G. HOLS. hese Proceedings, 17, 508, 19H. Comm. Leiden NO

2 1172 helium the change was too small for the wires to be used as thermometers. For echn. Pt a magnetic field of 150 gauss has practically no influence on the resistance. For Pt-Ag and Pt-r we established that a magnetic field of 150 gauss does not leave a permanent disturbance. able gives an extract from the data observed. OK. ABLE. RRo Pt-Ag Pt-r echn. Pt Brass Phosphor bron ze Brass shows down to the temperatures of liquid hydrogen a continual change of the resistance with temperature (see table ). n liquid helium however it los es its resistance almost entirely (see Fig. 1). Nevertheless thc residual resistance appears to be still dependent on the temperature. n a magnetic field of 150 gauss the resistance was considerably larger. A permanent disturbance in consequence of the influence of the magnetic field was not established. % Q010 QOOO t m J ~ ~ 2 3 Fig. 1. o Resistance without magnetic field. in 11 of 150 gauss.

3 1173 his particular behaviour of the resistance at the temperatures of liquid helium is probably due to a small impurity caused by tin, which could be detected with the aid of the spectrograph. Other supra conductors were not found with the spectrograph. Phosphor~bronze. Por the change of the resistance down to the tem~ peratures of liquid hydrogen see table. Prom 20.4 to 16.2 K. the resistance changes only very slightly. able Ha gives the results of the measurements in liquid helium of the wire Ph~br (for which also the data given in tab e are available). with icepoint~resistance 34.4 Q, made on Dec. 20th '28, and table b gives the results of the measurements in the cryostat for the lowest temperatures 1). made on May 31th '29. he temperatures are derived from the vapour pressure of helium, on the basis of the measurements of KEESOM and WEBER with N!2SRGAARD 2). and with SCHMD 3). ABLE Ha. Resistance of Phosphor-bronze ok. Magnetic field olf R.Ro Magnetic field on (150 gauss) Prom the data given in the tables Ha and Hb it appears that the resistance of the phosphor~bronze wire Ph~br changes pretty nearly linearly in liquid helium. he relative change in th is tèmperature reg ion is much larger 4) than has previously been obtained for any other ahoy: e.g. it is 1) hese Proceedings, , Comm. Leiden NO. 195c. 2) hese Proceedings, 32, 864, Comm. Leiden NO. 202b. 3) Soon to be published in these Proceedings. ") We Jeave open the question whether this particular decrease of the re si stance at the temperatures of liquid helium is due to a small admixture of a supraconductor or not. 76 Proceedings Roya Acad. Amsterdam. Vol. XXX

4 1174 ABLE lb. - Reslstance of Phosphor-bronze ok. RRo Qns %0 Q.7S Q725 Q70 Q y V p" V ~ V as25 U s 1 15 c Pig.2. UK 4 s a 20 Dec Reslstance without magnetic field. eb.. in of 150 gauss. G 31 May 1929 without ca. 55 times as large as for constantin. his was entirely confirmed by two other phosphor~bronze wires on Nov. 13th he change of the resistance appears to be quite reproducible. he influence of a magnetic field of 150 gauss appears to be relatively small and does not cause a lasting disturbance. t appeared that a phosphor-bronze wire (Ph-br 11), that was pre-

5 1175 annealed. behaved in an entirely different way. lts resistance did not appear to undergo th is important change at the temperatures of the liquid helium. n fact it differed by little from th at in liquid hydrogen. We conclude that phosphor-bronze. that has not been annealed. is extremely suitable for use as a secondary thermometer in the liquid helium range of temperatules. 76*

(Communicated at the meeting of January ).

(Communicated at the meeting of January ). Physics. -.. T he variatian at the dielectric canstant at liquid axygen with temperature." By WACLAW WERNER and W. H. KEESOM. (Communication N0. 178c from the physical laboratory at Leiden.) (Communicated