448 



is compared with lliat for oilier siiper-coiidiictors (Comni. N". 133r/, 

 p. 67) the retrogression of the limit caused by a greater decrease 

 of leinperature below the vanishing point tein|)eratiire would üeem 

 to be recognisable in the iiieasuiements of wires of ditferent iiielals, 

 as has been ascertained already by measurements of one wire of 

 one metal. But we must point out that this general conclusion 

 cannot be drawn before the value of the threshold cnrrent as a 

 function of 7\;u)ishing point— ^' a"d of the dimensions of the wire 

 is known and after it has l)een ascertained whether a returning 

 resistance is due to a single "bad place", or whether it is distributed 

 over the whole length of the wire. 



§ 5. Threshold value. At some temperatures we tried to determine 

 the treshold value of the current, that is the strength of the current 

 sent through the wire, which again generates a measurable potential 

 difference. The results are given in Table III. 



The two first observations in Table 111 show that for wires of 



i 

 different diameter at the same temperature the quantity - seems to 



r 



be much more a constant than the current density. The latter 



quantity occurs in the expression for the magnetic field at the surface 



of the wii'e through which a current passes. 



F. B. SiLSBEii ^) drew special attention to the influence of this field. 



The determination of the threshold value of the magnetic field for 



thallium by means of external fields, and the comparison of it with 



H, derived from the two first observations in Table 111 by means 



of H =z — , (the wires TZ-VIll and 7V-1X therefore being regarded as 

 r 



straight), must prove whether these two strengths of field are equal, 



and that therefore the magnetic field is the primary factor in the 



disturbance of super-conductivity. Then the "bad places" referi-ed to 



more than once, are the places with the smallest diameter; the returning 



of the resistance caused by the current, occurs first in these places 



only. The above mentioned experiment with thallium is prepared 



and also a similar one on a more extejisive scale with the more 



easily manipulated tin ; it must not be forgotten, that at the return 



of the resistance at great strengths of current such a development 



of heat soon takes place, that first the wire and if this melts the 



galvanometer is in danger; this makes the determination of laige 



current threshold values rather risky. 



1) F. B. SiLSBEE, Sclent. Pap. Bur. of Stand. No. 307 (1917). 



