687 



superconductive. Now that super-conducting plates of tin and lead can 

 be made, the experiments on this subject are made practicable, and 

 the plan of making- these has assumed a promising form, since I 

 have obtained the prospect of doing it with Leïjard himself, which 

 I highly value. If the potential phenomena are caused by local 

 disturbances, we may expect that in experiments with thin plates, 

 by a correct choice of the places to be experimented upon, they will 

 be of little importance. If, as might be imagined according to § 4, 

 the potential phenomena are connected with peculiarities in the 

 movements of the electrons, then they would be of prime impor- 

 tance in phenomena such as we have here under consideration. 



(f. The correspondence of the potential phenomena in tin and lead 

 to those in mercury is very striking. As regards tin, it was stated 

 already in § 13a, and further investigation has confirmed it and also 

 extended to lead. All the considerations with regard to them for the 

 case of mercury can thus immediately be applied to tin and lead. 

 On the other hand the latter may serve to elucidate the doubtful 

 points in mercury. 



With the bare tin wires at 4°. 25 K. measurements were made 

 which acquaint us with the amount of heat, given otï to the liquid 

 helium above the vanishing point; whether it is proportional to the 

 surface of the wire, as is to be expected, when the heat is mainly 

 given off to the liquid, could not be settled yet. With (he rolled 

 out tin wire, with which the various measurements were successful, 

 it was great, which corresponds to the fact that here tJie ratio 

 between the heat-conveying surface and the heat developed is very 

 favourable. It was estimated at 0.5 watt per 1 degree difference of 

 temperature. Still at i°.6 K., 1.4 microwatt caused a local rise of 

 temperature to the vanishing point. As in § 11 we deduce from 

 this that the whole development of heat is local. The hypothesis 

 that in this way "bad places" show themselves is confirmed by the 

 fact that through a wire like this at the boiling point of helium, 

 therefore above the vanishing point, a current of 9 amp. could be 

 sent, and all the Joui.e heat was absorbed by the liquid helium, 

 whUe with a current only a little stronger the wire gave way 

 (presumably by the forming round the wire of a vapour bubble in 

 the helium, which caused calefaction in the wire). 



The different threshold values for the bare lead wire and the 

 lead coil § 15, and for the bare tin wire and the tin coil § 14, 

 may throw light upon the influence of more or less easy conditions 

 of heat loss. The phenomena at the disappearance of the resistance 

 with the bare tin wire with sentinel wires make the hypothesis 



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