560 
ME. G. GOEE ON ELECTEOTOESION. 
shortening effect) occurred if the bar was in the coil, but not otherwise, and was lessened 
to 5 mm. if the undivided current was caused also to traverse the bar. The torsional 
movement did not change in direction on changing the course of the current either 
in the coil, the bar, or both, but in each case agreed with a diminution of diameter of 
the coil ; it was therefore of a different kind to the phenomenon of electrotorsion de- 
scribed in this paper. 
46. Electrotorsion of nickel. 
A bar of nickel 60 centims. long and 19 mm. diameter was subjected to the influence 
of simultaneous and divided axial and coil-currents from twelve Grove’s cells whilst 
in the axis of a suitable coil ; but only very minute torsional movements occurred, 
apparently in consequence of unsuitable dimensions of the bar. 
47. Electrotorsion in telegraph-wires. 
[As nearly all the overground telegraph-wires on the surface of the earth are com- 
posed of iron, and are more or less magnetized by terrestrial magnetic influence, especially 
those lying in the direction of the terrestrial magnetic meridian, it is evident that on 
the passage of every electric current through them electrotorsional movement tends to 
occur.] 
Note on Mr. Gore’s Paper on Electrotorsion. By Sir William Thomson, F.B.S.* 
In Section 5, “ General cause of the torsions,” the phenomena are attributed to the 
combined influence of ordinary magnetic polarity and the magnetic condition of iron 
at right angles to that. To see precisely how this combined influence produced the 
results discovered by Mr. Gore, we have only to look to Joule’s discovery of the effects 
of magnetism on the dimensions of iron and steel bars, and of the musical sounds con- 
sequently produced in an electromagnet every time battery-contact is made or broken. 
This great discovery was first described in public on the occasion of a conversazione 
held at the Royal Victoria Gallery of Manchester, on February 16th, 1842. A printed 
account of it is to be found in the eighth volume of Sturgeon’s 4 Annals of Electricity,’ 
p. 219, and in the ‘ Philosophical Magazine,’ 1847, first half year. The following are 
the chief results obtained by Joule: — 
1. When a wire or bar of iron (or steel) is alternately subjected to, and left free 
from, longitudinal magnetizing force, it alternately becomes longer and shorter. 
2. In the same circumstances its volume remains sensibly unaltered ; and therefore 
it experiences lateral shrinking to an extent equal to half the extension in length f. 
3. Joule verified the lateral shrinking by passing a current through an insulated 
* Not read before the Society, but ordered to be printed. 
j- I,t is understood, of course, that the shrinking is reckoned in proportion to the transverse diameter, and 
the extension in proportion to the length, as is usual in the geometry of strains and in the theory of elasticity. 
