445 



ill an atmosphere of helium proved to be completely sufficient; 

 the results of the zero point determinations are found in Table I 

 (see p. 444). The zero point measurements are partly due to Dr. 

 J. M. Burgers, now Professor at Delft. 



§ 3. Measurements in liquid helium; (letennination ofthevanisliing 

 point temperature. The resistances were placed in the cryostat 

 provided with a stirring apparatus shown in Comm. N'. i24c, 

 tig. 4. For determining the amount of their resistance the second 

 method mentioned in ^ 2 was used. The measurements were 

 always made with both directions of current in the circuit of the 

 resistances, care being taken that to each of them the direction 

 of the current in the compensation a[>paralus corresponded. More- 

 over, in measurements below the vanishing point temperature the 

 galvanometer was observed when the current was reversed in the 

 circuit of the resistances only (this betrays super-conductivily more 

 quickly): in the case of super-conductivity there must be no 

 change of position observable. 



The temperatures are determined by the measurements of the 

 vapour pressure of the helium bath, the connection between pressure 

 and temperature havijig been derived graphically from the results 

 in Comms. W. 119 and N". 1476. Close to the vanishing point 

 temperature the pressure of the bath was followed with the katheto- 

 meter (June 5^'' 1919); we give below the diagram of a series 

 of observations (in this field of temperature 1 m.m. pressure = about 

 0.0 1 of a degree). 



60.0 mm 

 59,0 mm. 



In spite of the fact that the wires were not in contact with the 

 liquid helium, in the measurement of their resistances the galvano- 

 meter reacted with surprising rapidity to the changes of tempe- 

 rature of the bath. The results are given in Table 11. 



From Table II it appears that a constant difference Lw exists for 

 all temperatures; in spite of this additive resistance ') of Tl-VUl-1916 

 with regard to Tl-\X-1916 both become super-conducting at the same 



1) If this additive resistance is taken constant, it becomes 0,00083 TFo= 0,00370; 

 we must assume in the meantime that it is largely due to the joint. 



