of various Metals. 345 



the numerical results obtained with iron, and do not consider 

 them worth publishing. To myself, however, they make it 

 seem probable that the rotational coefficient in iron is less in 

 a field of intensity 1000 than in a field of intensity 7500 ; and 

 I expect to prove this when I am able to take up the matter 

 again. Cobalt should of course be examined in the same way; 

 nor must it be forgotten that it is by no means proved, as yet, 

 that the non-magnetic metals will show a constant rotational 

 coefficient when tested between wide limits of magnetic force. 

 The object of another experiment was to determine, if pos- 

 sible, whether any part of the rotational effect could be made 

 permanent under favourable conditions. For this purpose a 

 piece of clock-spring was taken, tempered very hard, and then 

 reduced by action of nitric acid to a thickness of about '06 

 millim. This piece of steel was firmly imbedded upon a plate 

 of glass in a layer of cement made of melted bees-wax and 

 resin. This plate, with the usual electrical connexions, and 

 with a current flowing through it, was placed in the usual 

 position between the poles of the electromagnet ; the magnet 

 current was turned on, then off, and the plate removed from 

 between the poles in order to avoid the action of the very con- 

 siderable residual magnetism of the electromagnet. A reading 

 of the Thomson galvanometer in the transverse circuit was now 

 made ; then the plate was replaced between the poles and the 

 current turned on again, but in the opposite direction. The 

 magnet-current being again interrupted, the plate was again 

 removed from the field, and another reading of the Thomson 

 galvanometer was made. The two readings differed by several 

 centimetres on the galvanometer-scale. The experiment was 

 repeated, and always with a like result. 



There was no room for doubt that the direction of the equi- 

 potential lines in the steel was permanently changed by the 

 action of the magnet. This change was in the same direction 

 as the temporary change produced by the magnet's action, and 

 perhaps equal to 2 per cent, of the temporary change. 



This result was of course not unexpected. The hardened 

 steel must have become permanently magnetized transversely; 

 and this magnetization should produce an effect similar to 

 that of temporary magnetization. The experiment is of inter 

 est, however, as indicating that the rotational effect is not due 

 to the mere mechanical stress to which the metal is subjected 

 in the magnetic field ; for though no one has ever pointed out 

 how any such stress could produce the effect observed, many 

 have no doubt questioned whether it might not after all be due 

 in some obscure way to such stress. 



It may be stated incidentally that the transverse effect 



