Self-induction of Wires . 197 



exact *. [I may, however, here mention that in experiments 

 with mere fine copper-wire coils there are sometimes to be 

 found traces of variations of resistance-balance with the fre- 

 quency of interinittence, of very small amount and difficult to 

 elucidate owing to temperature-variations.] Balancing partly 

 by M 64 and partly by the iron cores, the residual sound in- 

 creases from zero with M 64 only, to the maximum with the 

 cores only. Halving the strength of current upsets the 

 induction-balance in this way. The auxiliary core must be 

 set a little closer when the current is reduced. This would 

 indicate a slightly lower inductivity with the smaller magne- 

 tizing force, and proves slight curvature in the line of induc- 

 tion. But, graphically represented, it would be invisible 

 except in a large diagram. 



It is confidently to be expected, from our knowledge of 

 the variation of p, that when the range of the magnetizing 

 force is made much greater, the ability of nonconducting iron 

 to act merely as an increaser of inductance will become con- 

 siderably modified, and that the dissipation of energy by 

 variations in the intrinsic magnetization will cease to be 

 insensible. But, so far as weak magnetizing oscillatory 

 forces are concerned, we need not trouble ourselves in the 

 least about minute effects due to these causes. Under the 

 influence of regular intermittences, the iron gets into a 

 stationary condition, in which the variations in the intrinsic 

 magnetization are insensible. It seems probable that fi 

 must have a distinctly lower value under rapid oscillations 

 than when they are slow. The values of /a calculated from 

 my experiments on cores have been usually from 50 to 200, 

 seldom higher. I should state that I define /n to be the ratio 

 B/H, if B is the induction and H the magnetic force, which is 

 to include h, the impressed force of intrinsic magnetization. 

 (See the general equations in Part I. f) It is with this //«, not 

 with the ratio of the induction to the magnetizing force as 

 ordinarily understood, that we are concerned with in experi- 

 ments of the present kind. 



Knowing, then, that iron when made a nonconductor acts 

 merely as an inductor, when we remove the insulation and 

 make the iron a solid mass, it requires to be treated as both a 

 conductor and an inductor, just like a copper mass, in fact, of 

 changed conductivity and inductivity. When the coil is a 

 solenoid whose length is a large multiple of its diameter, and 

 the core is placed axially, the phenomena in the core become 

 amenable to rigorous mathematical treatment in a compara- 



* < Electrician,' April 30, 1886. 

 t Phil. Mag. August 1880. 



