26 



Prof. D. E. Hughes. 



[Mar. 31, 



question to it, does the passage of electricity through a wire produce 

 a change in its structure ? the answer came, it does, and that to a 

 very considerable extent ; for an iron wire adjusted to perfect zero, 

 and which would remain free from any strain for days, becomes in- 

 stantaneously changed by the first passage of a current from a single 

 cell of Daniell's battery ; the wire has now a permanent twist in a 

 direction coinciding with that of the current, which can be brought 

 again to zero by mechanically untwisting the wire, or undoing that 

 which the passage of electricity has caused. Before describing the 

 new phenomenon, I will state that the only modification required in 

 the apparatus, is a switch or key by means of which the telephone 

 upon the wire circuit is thrown out of this circuit, and the current 

 from a separate battery of two bichromate cells passed through the 

 wire alone, at the same time, care being taken that no current 

 passes through the coil, but that its circuit should remain open during 

 the passage of the electric current through the wire under observation ; 

 an extra switch on this circuit provides for this. The reason for not 

 allowing two currents to react upon each other, is to avoid errors of 

 observation which may be due to this cause alone. When, however, 

 we take an observation, the battery is upon the coil and the telephone 

 upon the wire alone ; an experiment thus consists of two operations. 

 First, all external communications interrupted, and an electric current 

 passed through the wire ; and, second, the electric current taken off 

 the wire, and all ordinary communications restored. As this is done 

 rapidly by means of the switches, very quick observations can be 

 made, or if desired the effects of both currents can be observed at the 

 same instant. 



Now, if I place upon the stress bridge a soft iron wire J millim. dia- 

 meter, 25 centims. long, I find, if no previous strain existed in the wire, 

 a perfect zero, and I can make it so either by turning it slightly back- 

 wards or forwards, or by heating the wire to a red heat. If I now give 

 a torsion to this wire, I find that its maximum value is with 40° torsion, 

 and that this torsion represents or produces electric currents whose 

 value in sonometric degrees is 50 ; each degree of torsion up to 40 

 produces a regular increase, so that once knowing the value of any 

 wire, we can predict from any sonometric readings the value in torsion, 

 or the amount of torsion in the opposite direction it would require to 

 produce a perfect zero. 



If now I place this wire at zero, and thus knowing that it is entirely 

 free from strain, 1 pass an electric current through it, I find that this 

 wire is no longer free from strain, that it now gives out induction 

 currents of the value of 40, and although there is no longer any 

 battery current passing through this wire that the strain is permanent, 

 the outside coil neither increasing or diminishing ; the internal strain 

 it has received by the passage of an electric current through the wire, 



