Self-induction of Wires. 193 



quite insensible, or masked by another effect, so that we are 

 confined to iron and the other strongly magnetic bodies. 

 The foundation of the theory is Poisson's assumption (no 

 matter what his hypothesis underlying it was) that the 

 induced magnetization varies as the magnetic force ; and 

 when this is put into a more modern form, we see that 

 impressed magnetic force is related to a flux, the magnetic 

 induction, through a specific quality , the inductivity, in the 

 same manner as impressed electric force is related to electric 

 conduction-current through that other specific quality, the 

 conductivity of a body. Increasing the inductivity in any 

 part of the magnetic field of a coil, therefore, always increases 

 the inductance L, or the amount of induction through the 

 coil per unit current in it, and the magnetic energy, ^LC 2 . 

 The effect of iron therefore is, in the steady state, merely 

 to increase the inductance of a coil, without influence on its 

 resistance. I have, indeed, speculated* upon the existence of 

 a magnetic conduction-current, which is required to complete 

 the analogy between the electric and magnetic sides of 

 electromagnetism ; but whilst there does not appear to be any 

 more reason for its existence than its suggestion by analogy, 

 its existence would lead to phenomena which are not ob- 

 served. 



But this increase of L by a determinable amount — deter- 

 minable, that is, when the distribution of inductivity is 

 known, on the assumption that the only electric current is 

 that in the coil — breaks down when there are other currents, 

 connected with that in the coil, such as occur when the latter 

 is varying, the induced currents in whatever conducting 

 matter may be in the field. L then ceases to have any 

 definite value. But in one case, that of S.H. variation, the 

 mean value of the magnetic energy becomes definite, viz. 

 £L/C 2 , where 1/ is the effective L, and C the amplitude of 

 the coil-current, the change from ^ to J being by reason of 

 the mean of the square of a sine or cosine being \. This 

 definiteness must be, because the variation of the coil-current 

 is S.H., as well as that of the whole field. That L' is less 

 than L, the steady-flow value, may be concluded in a general 

 though vague manner from the opposite direction of an in- 

 duced current to that of an increasing primary, and its 

 magnetic field in the region of the primary; or, more dis- 

 tinctly, from the power of conducting- matter to temporarily 

 exclude magnetic induction. 



In a similar manner, the resistance of a coil, if regarded as 

 the R in EC 2 , the Joulean generation of heat per second, 

 * * Electrician,' January 4, 1885, p. 219 et seq. 



