ELECTRICAL PROPERTIES OF CONDUCTORS 589 



Hence an entirely new condition of matter has been dis- 

 covered, one, namely, in which an electric current will flow 

 along a wire of the material without any appreciable potential 

 difference between the ends. The condition is, however, not 

 at all a simple one, and although it is by no means fully 

 investigated, enough has been proved to show that much 

 further research is necessary before this super-conductive state 

 will be properly understood. It will be sufficient here to give 

 some of the details of two of the lines on which investigation 

 has proceeded, and some of the results and conclusions reached 

 in others. It is stated above that the critical value of the 

 temperature which marks the boundary between the normal and 

 the super-conductive states is only a constant for small currents. 

 Investigation on this point shows that at this temperature, and 

 at every one below it, there is a limiting value of the current 

 which must be observed if the super-conductive state is to be 

 retained. Above this value the current requires an appreciable 

 difference of potential to force it through, but one which seems 

 to be proportional to the excess of the current above the limiting 

 or " threshold value," as it is named by its discoverers. Hence 

 just above the threshold value, as, of course, below it, Ohm's 

 law does not apply, the deviation becoming proportionately 

 less and less for greater currents, but the exact relation is not 

 yet known. It is also not known yet whether the change at 

 the threshold value is due to a uniform variation of the state 

 of the whole conductor or whether it is due to some local 

 heating caused by some less conductive portion which raises the 

 local temperature above the critical at that point, and hence 

 produces a point for the production of Joule-heat, which changes 

 the whole condition of the conductor. To determine this will 

 require many and careful measurements with conductors arranged 

 so that the rate of withdrawal of heat can be regulated. Also 

 a significant fact was observed that with a lead wire in a 

 vacuum the current could rise far above the value which had 

 been found for the threshold value using a naked wire immersed 

 in liquid helium, which has been shown to be an excellent 

 insulator. 



The other factor which causes a variation in the state is 



the magnetic effect of the current itself. This also has only 



been investigated to a limited extent, but owing to its extreme 



importance and its apparent relation to the threshold value, 



39 



