Electrodynamic qualities of metals. 
73 
229. A very interesting discovery of Wiedemann’s (‘Galvanismns,’ §§491 and 498), at 
first sight, seemed to find its explanation in the aeolotropic difference of magnetic 
susceptibility which I have found to lie induced by aeolotropic stress in soft iron. The 
phenomenon consists in the development of longitudinal magnetization by twisting a 
wire through which a magnetic galvanic current is maintained longitudinally. The 
annexed double diagram, copied from Wiedemann’s hook,'”' describes the change of 
J/f/.l. fig, 2. 
ideal magnetic molecules which would represent the actually observed effect, which is 
that the end of the wire by which the current enters becomes a true north pole when 
the twist given to it is right-handed (or that of an ordinary screw). If this effect 
were due to greater susceptibility in one direction than in another, the direction of 
greatest susceptibility would be the direction sloping at 45°, upwards to the right in 
the front of the right-hand diagram ; that is to say, it would be the direction of positive 
pull in the stressed material. But the exceedingly intense magnetization by influence 
of circular lines of force round the cylinder, produced by currents of such strength as 
Wiedemann may be supposed to have used, must in all probability have been above 
the critical degree of magnetization at which the effect of pull becomes reversed, and 
therefore in all probability the direction of least susceptibility in the actual circum¬ 
stances must have been that of positive pull. Hence, it seems almost impossible to 
admit the explanation of Wiedemann’s result by aeolotropic magnetic susceptibility in 
the circumstances.t The true explanation is not easily conjectured : for another 
cause, also adverse to Wiedemann’s result, is operative. The electric conductivity 
of the iron is probably least in the direction of the positive pull and greatest in the 
direction of the negative pull in the stressed material.^ This aeolotropic quality in 
* I have altered the letters n and s of Wiedemann’s hook to Gilbert’s old wholesome rule of putting 
n to represent true northern polarity, or the polarity of the same name as that of the earth’s northern 
regions, and similarly s to represent true southern polarity. 
f This experiment has been repeated for me since the communication of this instalment to the Royal 
Society, by Mr. Macfarlane, with currents, not hitherto measured or estimated in absolute measure, but 
strong enough to greatly heat the iron wire. The result was always the same as Wiedemann’s, and was 
greatest with the strongest current used.—(W. T., May 22, 1878.) 
x “ Electrodynamic Qualities of Metals,” §§ 145—153 (W. Thomson, Transactions of the Royal 
Society, 1856). “On the Increase in Resistance to the Passage of an Electric Current produced in 
Certain Wires by Stretching” (Tomlinson, Proc. Roy. Soc., No. 183, 1877). 
MDCCCLXXTX. 
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