24:4 The Mathematical Electricians of the 



Two years later Thomson presented to the Koyal Society a 

 memoir* in which the results of Poisson'a theory of magnetism 

 were derived from experimental data, without making use of 

 the hypothesis of magnetic fluids ; and this was followed in 

 1850 by a second memoir,f in which Thomson drew attention 

 to the fact previously noticed by Poisson,J that the magnetic 

 intensity at a point within a magnetized body depends on the 

 shape of the small cavity in which the exploring magnet is 

 placed. Thomson distinguished two vectors ; one of these, by 

 later writers generally denoted by B, represents the magnetic 

 intensity at a point situated in a small crevice in the 

 magnetized body, when the faces of the crevice are at right 

 angles to the direction of magnetization ; the vector B is always 

 circuital. The other vector, generally denoted by H, represents 

 the magnetic intensity in a narrow tubular cavity tangential 

 to the direction of magnetization ; it is an irrotational vector. 

 The magnetic potential tends at any point to a limit which is 

 independent of the shape of the cavity in which the point is 

 situated ; and the space-gradient of this limit is identical with 

 H. Thomson called B the " magnetic force according to the 

 electro-magnetic definition," and H the " magnetic force accord- 

 ing to the polar definition " ; but the names magnetic induction 

 and magnetic force, proposed by Maxwell, have been generally 

 used by later writers. 



It may be remarked that the vector to which Faraday 

 applied the term " magnetic force," and which he represented 

 by lines of force, is not H, but B ; for the number of unit lines 

 of force passing through any gap must depend only on the gap, 

 and not on the particular diaphragm filling up the gap, across 

 which the flux is estimated ; and this can be the case only if the 

 vector which is represented by the lines of force is a circuital 

 vector. 



* Phil. Trans., 1851, p. 243 ; Thomson's Papers on Elect, and Mag., p. 345. 



t Phil. Trans., 1851, p. 269 ; Papers on Elect, and Nay., p. 382. 



I Of. p. 64. 



Loc. cit., 78 of the original paper, and 517 of the reprint^ 



