336 H. E. J. G. du Bois on 



physical methods. In doing so I shall not so much use the 

 conception of lines of force, but shall rather start from the con- 

 sideration of magnetization. For it is the latter quantity, 

 which for experimental reasons (Phil. Mag. [5] xxix. p. 303, 

 1890) I believe must be taken as the fundamental one from 

 the physical standpoint, much more so than induction, to 

 which such importance has lately been attached. 



The idea of an analogy (even when only in mathematical 

 treatment) of magnetic systems with other systems of fluxes 

 (hydrokinetic, thermal, electric) dates back as far as Euler. 

 Faraday (and following him Maxwell) and Sir W. Thomson 

 then each developed it in his own way. It has been worked 

 out and practically applied, however, during the last de- 

 cennium. 



Bosanquet (1883), Eowland (1884), W. v. Siemens (1884), 

 Gisbert Kapp (1885), especially consider the analogy with 

 the flow of electricity, and accordingly apply Ohm's law to 

 magnetic circuits. As, however, this law essentially implies 

 the resistance being constant, independent of the current 

 flowing, the above extension of its most characteristic meaning 

 can hardly be conceded from the physical point of view ; 

 nothing is thereby meant to be said against certain practical 

 advantages gained by introducing an (essentially variable) 

 magnetic resistance. Quite lately, again, Pisati (1890) has 

 laid much stress upon the analogy with thermal circuits, and 

 accordingly has applied Fourier's law to certain magnetic 

 systems. This appears perhaps less objectionable in so far as 

 thermal conductivity is not necessarily a constant as its elec- 

 tric analogue is, though Fourier originally introduced the 

 former as such. But even then it depends upon temperature, 

 not upon its space-variation or upon the flow of heat : the 

 analogy is therefore in no case a complete one, magnetic con- 

 ductivity being of course independent of magnetic potential. 



No physical objection, however, appears to exist to the 

 totally different treatment the question received at the hands 

 of J. and E. Hopkinson (1886) : they started from two safely 

 established mathematical propositions, and only made some 

 auxiliary assumptions in order to simplify the (approximate) 

 calculations. They so arrived at their graphic construction, 

 now used to a certain extent in machine design, and which 

 will be reverted to below. 



Poisson's old theory of magnetic induction, extended by 

 Neumann and Sir W. Thomson, as laid down e. g. in Maxwell's 

 treatise, is known to rest on the fundamental assumption of 

 constant susceptibility ; it therefore applies to all substances 

 except " ferromagnetics " (iron, cobalt, nickel, magnetite, and 

 any substance which may yet be found to behave similarly). 



