silsbee: conduction at low temperatures 599 



other turns, which is very similar to that due to an entirely ex- 

 ternal electromagnet. Consequently, when this field reaches the 

 critical value, first the inner turns will become resisting and, as 

 the current is increased, more and more of the wire will cease to 

 be superconducting. Because of the enormous factor by which 

 the conductivity decreases from the superconducting to the nor- 

 mal state, most of this decrease will take place when only a small 

 fraction of a turn of the coil ceases to be superconducting. Owing 

 to the cumulative effect of the successive turns, the field produced 

 by a given current is much greater in the coil than in the same 

 wire when straight, and, consequently, the current required to 

 give the critical field strength will be much less. This is verified 

 by the results of Onnes 5 on coils of lead and tin wire for which 

 the critical currents were, respectively, T V and £ of those for 

 the same wire when straight. No attempt has yet been made 

 to measure the further gradual increase of resistance, which 

 would be expected on this theory, as the current is further in- 

 creased and more and more turns become resisting. 



In the case of a straight wire of circular section the effect to 

 be expected is rather more complicated. Consider a supercon- 

 ducting wire of radius r carrying a current /, uniformly distrib- 

 uted over the cross-section. The magnetic field intensity H 

 at any point, distant r from the axis, but inside the wire, is given 



by 



H= 2 4 CO 



rl 



and that at the surface of the wire by 



#o=- (2) 



r 



If the current be increased to a value slightly greater than 



H c r 



——- , where H c is the critical field intensity for the material, the 



outermost layer of the wire will become resisting. Since this 



5 Comm. Phys. Lab. Leiden, No. 133, pp. 57, 60. 



