178 Lord Rayleigh on the 



E is really a store of energy. From the fact that the mag- 

 netism may be got rid of by very moderate tapping, we may 

 infer, I admit, that some energy is necessarily dissipated in 

 passing from the magnetized to the unmagnetized condition, 

 but the dissipation may be exceedingly small ; and the argu- 

 ment is not conclusive, since the mechanical energy of the 

 vibrations may be involved in the process. If we attempt to 

 demagnetize the iron in a straightforward manner by the 

 application of a reversed force, following the course indicated 

 by EFGO, then, so far from recovering, we actually expend 

 energy — that, namely, represented by the area EFGrOE. For 

 practical purposes, at any rate, it would seem that magnetized 

 iron cannot be regarded as the seat of available energy. 



The opposite opinion, which is widely entertained, appears 

 to depend upon insufficient observance of the distinction, vital 

 to this subject, between closed and unclosed magnetic circuits. 

 It is not disputed that available energy accompanies the mag- 

 netization of a short bar of iron, but this is in virtue of the 

 free polarity at the ends. The work stored is in fact that 

 which might be obtained, were the bar flexible, by allowing 

 the ends to approach one another, under their mutual attrac- 

 tion. When this operation is finished, so that the bar has 

 become a ring, there is no longer any work to be got out of 

 it, though it remains magnetized. 



In further illustration of this matter, reference may be 

 made to some interesting observations by Elphinstone and 

 Vincent * on closed magnetic circuits. As is well known, the 

 armature of a horseshoe electromagnet remains strongly 

 attracted after cessation of the battery-current. If, even after 

 a considerable interval of time, the coils of this electromagnet 

 were connected with those of a second electromagnet also 

 provided with an armature, and the first armature were then 

 violently pulled away, attraction set in and persisted between 

 the second armature and its electromagnet, the magnetism of 

 the original circuit being as it were transferred to the second. 

 Or, if a galvanometer were substituted for the second electro- 

 magnet, a deflection followed the forcible withdrawal of the 

 armature. In these experiments the necessary energy is 

 obtained, not from the magnetism of the closed circuit, but 

 from the work done in opening it, that is in pulling away 

 the armature. 



These considerations lead me to differ from Prof. Ewing 

 when he says f : — " In connection with ' secondary generators' 

 and induction-coils generally, the bearing of the first part of 



* Proc. Roy. Soc. vol. xxx. p. 287 (1880). 

 f I. c. § 34, p. 554. 



