453 



tap K is closed, the helium vapour formed will quickly drive the 

 liquid helium out of (he bell-shaped ci^yostat space; by opening the 

 tap /Tand putting on the electric heating in the spiral i^, a constant 

 va|)our stream may be sent through (he crj'ostat; the stream may 

 be brought (o the temperature desired by electric heating of the 

 spiral (jr ; thus the liquid level of the evaporating helium remains 

 between F blwo. G. The copper mantle ^inside the bell contributes to 

 the acquiring of an even temperature over the whole space; further 

 experiments must show in how far uniformity of temperature has 

 been achieved with the arrangement as described. The first cooling 

 uses a great deal of liquid helium. 



^ 3. Resistance and temperature determinations. 



The resistances are measured by comparison of the deflections of 

 the galvanometer, when connected with the extremities of an unknown 

 and a known resistance (0,001 or 0,01 5i 0. Wolff) ; the resistances 

 are proportional to the means of the deflections for both directions 

 of the current, as follows from the comparison of the deflections 

 for O.OOJ and 0,01 £i. 



The temperatures are detei-mined with a heliumgasthermometer 

 of constant volume and with open manometer, the height of the 

 barometer is read from an aneroid. In the measurements of May 

 18''' 1920 the zero pressure of the thermometer was calculated to 

 be about 1140 cm.; as it was not easy to determine this pressure 

 accurately, the pressure at the temperature of liquid helium was 

 taken as calibration point (this temperature followed from the vapour 

 pressure of the bath). 



For the measurements of May 28''' 1920 the zero-pressure of the 

 thertnometer was decreased to 290 cm., in order to have less difficulty 

 with the corrections on the provisional international Kelvin scale, 

 these corrections in and below the field of liqui(^ hydrogen being 

 insufficieTïtly known. As two calibration points the tensions of the 

 thermometer served, placed in liquid helium (May 28''' 1920) and in 

 liquid hydrogen (May 29'^ 1920); the temperatures of these points 

 again follow from the vapour pressure of the bath, using the data 

 from Comm. N». 1476 and W. 1566. 



For the correction of the indications of the thermometer on the 

 provisional international Kelvin scale, we had at our disposal the 

 data of Comm. Suppl. N'. 34^?, p. 17, note 4 (obtained from the 

 data of Comm. N'. 102c), in which jB_254°c. has been taken zero, 



