1383.] Theory of Magnetism based on New Researches. 195 



neutral ; ordinary hoop iron requiring generally eight double right 

 and left elastic torsions before it is nearly neutral ; there will be, 

 however, still a slight residual magnetism, which can be instantly 

 rendered neutral by a slight torsion given to the rod when held 

 vertically; or we may reverse the residual magnetism by the same 

 directive influence of the earth's magnetism. 



I have found it convenient to attach two brass clamp keys to the 

 extremities of the rods, or simply turn the ends at right angles, as 

 shown in the following diagram. 



Wertheim, 1857, remarked that if we magnetise a wire under 

 torsion, there will be a diminution upon freeing it from torsion, a 

 still greater upon giving it a contrary torsion ; and that its original 

 force is in a measure restored by returning to the torsion under which 

 it was magnetised, designating this phenomenon as " La rotation du 

 maximum de magnetisme," and it is difficult to explain this phe- 

 nomenon except upon the hypothesis of molecular rotation. 



Wiedemann discovered that a wire becomes magnetic on or after 

 the passage of a current, and in his " Galvanismus " he points out its 

 molecular character. Sir W. Thomson* expresses his opinion that the 

 effects are due to the outside twist of the wire forcing the current to 

 pass round a fixed centre, and, consequently, that the effects can be 

 explained as ordinary electromagnetism. This view evidently takes 

 notice of the spiral action of the current alone. I have repeated 

 this experiment, and find that the magnetism increases directly as the 

 electromotive force, so it would be difficult to infer that the action 

 of the current is due to molecular rotation, if it was not for the fact 

 that the wire is magnetic after the passage of the current, as can be 

 rendered visible by torsion, consequently I believe Wiedemann was 

 fully justified in regarding this effect as one of molecular rotation. 



Dr. Hooke, 1684, remarked that steel or iron was magnetised when 

 heated to redness and placed in the magnetic meridian. I have 

 slightly varied this experiment by heating to redness three similar 

 steel bars, two of which had been previously magnetised to saturation 

 and placed separately with contrary polarity as regards each other, 

 the third being neutral ; upon cooling, these three bars were found to 

 have identical and similar polarity. Thus the molecules of this 

 * " Phil. Trans.," 1879, p. 55. 



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