519 



that the experiment could be made equally well with the windings 

 "parallel" as it had been made with the windings in "series". A 

 calculation (by estimation and further proceeding in the same way 

 as with the coil) about the experiment with a lead ring of an internal 

 radius of i.2 cm. of a thickness of 0.3 cm. and of a width of 

 0.35 cm. and assuming that the threshold value found for the thin lead 

 wire would also hold for the thick i-ing, showed me, that it might 

 succeed very w^ell. ' 



This proved to be the case. The current of 320 anij». that was 

 registered in the ring remained constant for half-an-hour to l'/,, hence 

 the current density of 30 was in this experiment not much smaller 

 than it had been in one of the experiments with the coil of lead 

 wire, ^'iz. 49. This may for the present be regarded as a confirm- 

 ation of the supposition that the threshold value of current strength 

 of a conductor is mainly a threshold value of current density for 

 the material of the conductor. 



be subjected to a magnetic (ield which was to be removed afterwards — was also 

 applied in my experiments for the purpose of obtaining persisting currents in supra- 

 conduclors, and in the above last experiment actually with a ring as the conduct- 

 ing circuit. 



At the time I was so much occupied with the investigation of the peculiar laws 

 of electric conduction in mercury below the vanishing-point and of the degree to 

 which currents might be realised in resistanceless circuits without electromotive 

 force, that I had not yet attacked or was able to fully go into the problems relating 

 to currents to be generated in closed supra-conductors by induction (amongst which 

 problems that of the imitation of diamagnetic polarisation was an obvious one). 

 Still Mr. TAUuiiN Chabot's letter was the cause of my coming even then to the 

 conclusion, that in order to be able to obtain persisting currents outside the magnetic 

 tield by induction, an artitice based on the peculiarity of supra conductors was 

 required. As such I then found, that the cooling which is to make the conductor 

 supra-conducting is not applied, until the conductor is in the tield which 

 is to be used for the induction. Afterwards it was found, that by utilizing the 

 knowledge of the threshold values of current and tield circumstances may be realized, 

 in which a permanent current may be obtained outside the tield by induction on 

 a circuit which has been made supraconducling by cooling before the tield is applied. 



