598 silsbee: conduction at low temperatures 



tin, and 6° K for lead), 2 however, there is a sudden break in the 

 curve connecting resistance and temperature, and within a 

 temperature range of a few hundredths of a degree the resist- 

 ance drops from about 10~ 3 times its value at 0°C. to less than 

 10~ 10 times the same value. Other metals, such as gold, silver, 

 platinum, and iron, do not show this phenomenon but tend to 

 approach a constant value as the temperature is lowered to the 

 lowest value (1?6 K) at which such measurements have been 

 made. The critical temperature at which the change occurs is 

 very definite when the current used to measure the resistance is 

 small, but when the measuring current is very large the critical 

 temperature is found to be definitely lower. Conversely, if the 

 temperature of the bath be held constant some degrees below the 

 critical value and the current be increased, a certain "threshold" 

 value of current will be found at which the resistance suddenly 

 appears. 3 The lower the temperature the greater the value of the 

 critical current. 



It is further found that when a superconductor is placed in a 

 weak magnetic field it remains superconducting; but that, as the 

 field is increased, the normal resistance appears suddenly at a 

 certain critical value of the magnetic field, and for still higher 

 values of the field it increases slowly with the field. 4 The criti- 

 cal value is slightly less when the field is transverse to the direc- 

 tion of current flow than when it is longitudinal, but the differ- 

 ence is not great. 



The particular point which is the subject of this note is that 

 the "threshold" value of the current is that at which the magnetic 

 field due to the current itself is equal to the critical magnetic field. 

 In other words the phenomenon of threshold current need not 

 be regarded as a distinct phenomenon, to be explained by heating, 

 or otherwise, but is a direct result of the existence of the pheno- 

 menon of threshold magnetic field. 



If the specimen is in the form of a coil of wire it is evident that 

 the inner turns are in a magnetic field, due to the current in the 



2 Comm. Phys. Lab. Leiden, No. 133, pp. 7, 52, 60. 



3 L.c, p. 3. 



4 Comm. Phys. Lab. Leiden, No. 139, pp. 65-71. 



