Galvanometer Deflection. 407 



This ma j be sufficient so far as the subject is connected with 

 galvanometiy merely. I need hardly call attention to the im- 

 portance of these phenomena in relation to experiments such 

 as those of Dr. Schuster, where alternating currents are used. 



In attempting to get a clearer insight into the nature of the 

 effect on the magnetism of the needle, I was led to make some 

 numerical verifications, and to give some variations to the 

 experiment which may perhaps be of interest. 



The phenomenon of magnetic induction is more or less 

 complicated according to circumstances. We have to deal, 

 in fact, with several distinct phenomena, which may for the 

 present purpose be classified under two heads : — I. Tempo- 

 rary Magnetism ; II. Residual Magnetism and its gradual 

 decay. These phenomena are analogous to (1) the temporary 

 strain, (2) the permanent set and "elastic recovery"* in a 

 solid subjected to stress in any way. 



In what follows I shall assume that we have to do with 

 temporary magnetism merely. Even when thus simplified 

 the present case is to some extent peculiar. It is probable 

 that the maximum magnetization producible by a given force 

 is attained only after the force has been in action for some 

 time f . Now, if the effects we are considering be due to in- 

 duced magnetism at all, it is obvious that a very considerable 

 fraction of the induced magnetism due to a given magnetic 

 force must be developed in an interval of time incomparably 

 smaller than the ^y of a second, whereas in the ordinary 

 experiments on induced magnetism the time allowed for the 

 development of the magnetization is practically unlimited. 

 This peculiarity gives the present case additional interest. 



I shall, in what follows, assume that when the magnetic 

 force to which an element of iron or steel is subject varies, 

 the corresponding variation of the magnetic moment of the 

 element follows at an interval of time which is incomparably 

 shorter than any other we have at present to deal with (e. g. 

 the time during which either of the induced currents remains 

 in the neighbourhood of its maximum). 



With this assumption, we may apply the ordinary theory of 

 magnetic induction. Three general conclusions may at once 

 be drawn from the simple consideration that an elongated 

 body tends to place its axis parallel to the lines of magnetic 

 force : — 



* By elastic recovery is meant what in Germany is familiarly known 

 under the name of " Elastische Nachwirkung." I do not know of any 

 English name for it which has the sanction of good authority. 



t Wiedemann, Galvanzsmw, Bd. II. 2, p. 100. Also Faraday, Phil. 

 Mag. [IV.] vol. ix. p. 92. 



