w. 



i4 



Physics. "Furfh('F experimenU ivltk Ur/itkl helium. L. The persistence 

 of currents /vithont electro-niotlve force in supra-conductimj 

 circuits" (Couliiiiuition ofJ). B}- Prof. H. Kameklincjh (Jnnes. 

 Conimiiiiieation N". 141 /> from the Physical Laboratory at 

 lieideii. 



(Communicated in the meeting of June !27, 1914). 



§ 9. The iweservntiov of an olectro-ldnetio momentum. All the 

 phenomena that were dealt with in the preceding sections (J) ^) 

 showing the persistence of the magnetic moment of the coil, 

 Avithont the action of an electro-motive force, agree Avith what was 

 deduced on the snpposition that a cnrrent ilows through the coil of 

 the value calculated, and which diminishes according to the time 

 of relaxation calculated. At the same time, it was desirable to have 

 a conclusive proof that the magnetic moment of the coil is really 

 cansed by a current. We should then be able to prove conversely 

 by the continnation of the moment, that the time of relaxation of 

 the curi'ent is very long, and a value, or otherwise an upj)ei limit 

 could be given for the micro-residual-i'esistance of the conductor in 

 which this current tlows. 



I got this proof in the following manner. 



On either side of the |)laco, whei-e the ends of the windings of 

 the coil are sealed together and close lo it, two wires ti, /> (see 

 fig. 2 and 1) were tixed which lead to a l)allistic galvanometer. 

 Between these points of attachment ihe cui-rent can be cut through 

 under helium, by pulling up by a thread a bronze K)op j)i'0\ided on 

 the inside with a knife edge at m (see tigs 3 and J . Figs 1 and 2 give 



1) Disregarding the existence of Ihresliold-values of current and field and consi- 

 dering that, below these, supra-conduclors add up algebraically without appreciable 

 loss the inductional impulses which act on them in the course of lime, two points 

 of view may be very simjily contrasted in connection with the experiments so far 

 described on the production of currents persisting for a long time. 



The first is analogous to lliat taken up in Wlber's explanation of diamagnetism. 

 in this case we deal with supra-conducling circuits which are currentless outside 

 the magnetic field. By ])ringing those into a field currents may be obtained which 

 persist as long as the field remains unchanged. But when the field disappears the 

 circuits become again free of current, hi this manner a good imitation is obtained 

 of diagmagnetic polarisation. The other point of view may be called the antilo- 

 gon of that of Weber. We provide in a magnetic field supra-conducting circuits 

 which are free of current. When these circuits are brought outside the field, they 

 show a current persisting for a long time. Outside the field they imitate permanent 

 magnets. It must, bowever, not be lost sight of, that this imitation is in so far 

 incomplete, as when the circuits are brought back into the field, they return to 

 the currentless condition. 



