562 
MR. G. GORE ON ELECTROTORSION. 
where a denotes the radius of the wire, and r denotes distance from its axis. (The 
same description, as is now well known — thanks to the beautiful illustrations and 
diagrams of iron-filings by which Faraday showed it in the Royal Institution — is 
approximately applicable to the field of force in the neighbourhood of a straight por- 
tion of wire conveying, a current, provided no other part of the wire is near.) Hence 
the intensity of the magnetization of the substance is equal to 
2 y 
where denotes the “ magnetic susceptibility” * of the substance. Let now a uniform 
magnetizing force X be applied along the whole length of the wire. This, combined 
with the force due to the current through the wire, gives at any point of the substance 
a resultant force equal to ,y/ in a direction inclined to the length of the 
wire at the angle whose tangent is The lines of force in the resultant field are 
fir A. 
therefore spirals. The wire being supposed infinitely long, the magnetization will still 
be poleless f, and will be everywhere in the direction of the resultant force ; and its 
intensity will be equal to the resultant force multiplied by the magnetic susceptibility. 
The extension of the substance along the spiral lines of magnetization, and its shrinkage 
along the orthogonal spirals, to be anticipated from Joule’s old results, give rise to 
Gore’s phenomena of electrotorsion. 
Although we thus see Gore’s “ electrotorsion ” as a geometrical consequence of the 
earlier discovery of Joule, we must, nevertheless, regard Mr. Gore’s investigation as 
having led to an independent discovery of a remarkably interesting character, enhanced 
by the well-designed and necessarily laborious working out of varied details described 
in his paper. 
It is difficult to conceive any physical investigation, except Faraday’s magnetic rota- 
tion of the plane of polarization of light, more important towards a physical theory 
of magnetism than Joule’s result (No. 5) above. It suggests an interesting extension 
of Mr. Gore’s investigation. Let the wire rod or tube experimented upon be stretched 
by a heavy weight, and at the same time subjected to a constant twisting-couple of 
sufficient magnitude, then, no doubt, the torsions as well as the elongations observed 
under varying magnetic influences will be the reverse of those discovered by Mr. Gore. 
The investigation ought, of course, to be varied by applying couple alone and longitu- 
dinal pull alone. 
* Thomson’s reprint of ‘ Electrostatics and' Magnetism,’ § 610. 3. 
f The free polarity in the actual experiments due to the finiteness of the iron bar or wire and of the mag- 
netizing helix reduces somewhat the magnitude of the effects, but does not alter their general character. 
