13 



problems, which were also of value for the knowledge of the con- 

 ditions which had to be considered, that I arrived at the simple 

 experiment which 1 am now able to describe, and which confirms 

 what I have adduced in a convincing way. 



For this experiment a conductor was available whose constants, 

 in so far as they were needed in designing the experiment, were 

 known : I refer to the coil of lead wire Pf>xn which has several 

 times been mentioned in |n-evioii,s papers. A thousand turns of lead 

 wire of Vm sq. mm. in section are wound on a small brass tube 

 oi 8 mm. in diameter in a layer 1.1 cm. thick and 1.1 cm. long. 

 'At the ordinary temperature the coil lias a resistance of 734 i2 and 

 as the inductance is 10 inilli-henries, the relaxation time may be put 

 at about 1:70000 of a second. The micro-residual resistance at 1°.8 K. 

 had been found to be more than 2 x 10^° times smaller than the 

 resistance at tlie ordinary temperature ; the relaxation time therefore 

 must be at least of the order of a day. The limit to which the current 

 may be raised before ordinary resistance is suddenly generated, had 

 also been determined; at 1°.8 K, this limit was (). 8 amp.; it is clear 

 that a lower current than that is sufticieut to make the coil into a 

 powerful little magnet. Finally the threshold value of the n.iagnetic 

 field, below which no resistance is produced in the coil was known : 

 at 1°.8 K. it had been found to be about 1000 gauss. It was ascer- 

 tained (cf. § 3), that it was unnecessary to use a field of that strength 

 to be able to make the experiment by means of generating a current 

 by induction in the conductor. The conductor after having been tested 

 as to its superconductivity had to be closed in itself in a supercon- 

 ductive way. This was effected b\ fusing the ends of the lead wire 

 together : in previous experiments it had been f()und, that this treat- 

 ment did not lead to the production of ordinary resistance. In view 

 of all the data I could be assured, that all the conditions necessary 

 lor the success of the experiment were fulfilled. 



§ 2. Arrangement of the experiment. The coil was fitted up in 

 the same cryostat which had served for the previous experiments 

 with the plane of the windings vertical in such a manner, that it 

 could be raised and lowered, as well as turned round a \'ertical 

 axis. Fig. 1 shows the arrangement diagrammatically. 



As the coil was closed the current in it was generated by induction. 

 A large WKiss-electromagnet, at hand for the experiments ofComm. 

 N". 140(/ could be moved on casters towards the cryostat to a position 

 in which the cryostat with the coil was in the interferruni. 



In order to obtain an unambiguous result it is advisable to be 



