its moment, a new coi! of 050 turns was wound, in wiiicli tli6 

 possibility of short-circuitin*^- was excluded by insulation of the 

 windings by picein and paper. It is true that the magnetic 

 properties of these materials are not known, but from the extinction 

 immediately above the boiling-point of helium of ))ersisting current 

 which was found in the course of (he experiments it is almost certain, 

 that the phenomena are entirely due to the lead. It was ascertained 

 that this coil was superconductive, which was a welcome result also 

 for the reason that the wire had been manufactured by compression, 

 and this process gives a much better guarantee of getting the same 

 product again by using the same method, and therefore of obtaining 

 beforehand the certainty of the wires prepared in this way being 

 supra-conducting. In making the experiment with 200 gauss at 2°.4 K. 

 a residual etfect of the same order as with Pbxii was found, but 

 smaller. The principal current was 0.5 and the residual current 

 0.020 Amp. It becomes ])robable, wiien these figures are compared 

 with those found with POxn , tl'^^l i» ^lie latter there really is some 

 short-circuiting, but there is also apai-t from the effect due to the 

 short-circuited windings a moment caused by tlie lead. 



It seems as if in the mean time this may be attributed to circular 

 currents in the lead of the wire, which are possible owing to the 

 wire having a certain thickness. We must distinguish in the wire 

 between an inside which is turned towards the axis of the coil, 

 and an outside. In the wire, even when the circuit is opened, a 

 current arises, in which the electricity passes along the whole 

 length of the windings on tlie outside of the wire (that is not closed 

 in itself), in order to turn round at the one end of the wire and 

 go back along the internal side. With induction in the closed circuit 

 this current is superposed upon the mean electric movement in 

 the circulating current, so that in the wire there is say a stronger 

 current on the outside, and a weaker on the inside. If by means 

 of a galvanic cell a current is sent through the wire, the same 

 phenomenon arises through the action of the lield of the current 

 itself. We are here evidently dealing with persisting Foucaui.t- 

 currents ^). 



§ 11. A .ntpraconductmc/ l-ey. In the experiments so far de- 

 scribed the supra-conducting closing of the conductor tested for supra- 

 conductivity was obtained by melting the two ends together. Now 



1) Several of the well known experiments hy Elihu Thomson with alternating 

 currents coulil also he made with parallel currents and supra-conducting experi- 

 mental objects. 



