684 



all metals, or at least a class of tliem, if ihey can be procured 

 snflicientiy pure, pass into the super-conducting state when reduced 

 to a low enough temperatui-e. Perhaps in all it wc-uhl also be 

 suddeidy. But the additive admixture-resistance which can be caused 

 by mere traces of admixtures, will in general make the detection 

 of the phenomena a difticuit one. 



[3. A number of experiments with resistance-free conductors of 

 which several suggest themselves at once, now that we can use the 

 easily workable super-conductors tin and lead, can be undertaken 

 with good prospect of success^). 



In this way the preparing of nonresisting coils of wire, with a 

 great number of windings in a small space, changes from a theo- 

 retical possibility into a practical one. We come to new difticulties 

 when we want not only to make a nonresisting coil, but to supply 

 it as a magnetic coil with a strong current *}. 



I have been engaged for some time making a preliminary estima- 

 tion of these difticulties''). 



The coils mentioned in § 14 and ^ 15 were made chiefly for 

 this purpose. The first of tin wire insulated with picëin, contained 

 on 1 cm. length in a layer of 7 mm. thickness 300 windings of 

 7j„ mm\ section (the resistance at ordinary temp, was 79 £i ). 

 While a current of 8 amp. could be sent through the wire before 

 it was wound when immersed in liquid helium, without reaching 

 the threshold value of current density (see § 14) the coil came to 

 the threshold value at J .0 amp. The number of ampere windings 

 per cm', of a section through the axis was about 400. The second 

 coil was wound of lead wire of V70 mm\ section, and contained in 

 a length of 1 cm. 1000 windings in a layer of 1 cm. thickness. 

 The resistance at ordinary temperature was 773 i2. The insulation 

 of the wires in each layer was obtained by silk threads, between 

 the different layers a thin piece of silk was placed. I thought that 

 the liquid helium penetrating into the coil through this texture would 

 cause the heat to be given off more easily all over the coil, while 



i) In our first paper about the disappearance of the resistance of mercury we 

 mentioned that this opened a new field of experiment. That mercury is liquid at 

 ordinary temperature was, however, a serious hindrance to entering it. 



3) A coil of this kind one would wish to place in the interferrum of a very 

 large electromagnet of Weiss, in the same way as the auxiliary coils contemplated 

 by him, in order to further raise the field. The field that is added by the coil 

 would in that case have to be greater than what would be sacrified by enlarging 

 the interferrum to make room for the cooling appliances. 



8) A possible difficulty was pointed out in note 2 § 4. 



