Self-induction of Wires. 195 



The same applies to the conductor forming the coil-circuit 

 itself ; it also may be regarded as having the magnetic dis- 

 turbance diffused into its interior from the boundary, and we 

 have only to make the coil-wire thick enough to make the 

 effect of the approximation to surface-conduction experi- 

 mentally sensible. But in common fine-wire coils it may be 

 wholly ignored, and the wires regarded as linear circuits. 

 There is no distinction between the theory for magnetic and 

 for non-magnetic conductors ; we pass from one to the other 

 by changing the- values of the two constants, conductivity and 

 inductivity. Nor is there any difference in the phenomena 

 produced, if the steady state be taken in each case as the basis 

 of comparison. But, owing to copper having practically the 

 same inductivity as air, there seems to be a difference in the 

 theory which does not really exist. 



A fine copper wire placed in one (say in branch 3) of a 

 pair of balanced coils in the quadrilateral, under the influence 

 of intermittent currents, produces no effect on the balance. 

 Its inductivity is that of the air it replaces, so that the steady 

 magnetic field is the same ; and it is too small for the diffu- 

 sion effect to sensibly influence the balance. On the other 

 hand, a fine iron wire, by reason of high inductivity, requires 

 the inductance of the balancing-coil (say in 4) to be increased. 

 The other effect is small in comparison, but quite sensible, and 

 requires a small increase of the resistance of branch 4 to balance 

 it. A thick copper wire shows the diffusion effect ; and if 

 we raise the speed and increase the sensitiveness of the 

 balance, its thickness may be decreased as much as we please, 

 if other things do not interfere, and still show the diffusion 

 effect. If thick, so that the disturbance is considerable, the 

 approximate balancing of it by change of resistance is insuffi- 

 cient, and the inductance of coil 4 requires a slight decrease 

 or that of 3 a slight increase. A thick iron wire shows both 

 effects strongly : the inductance and the resistance of branch 3 

 must be increased. These effects are greatly multiplied when 

 big cores are used ; then the balancing, with intermittences, 

 at the best leaves a considerable residual sound. The in- 

 fluence of pole-pieces and of armatures outside coils in 

 increasing the inductance, which is so great in the steady 

 state, becomes relatively feeble with rapid intermittences. 

 This will be understood when the diffusion effect is borne in 

 mind. 



If the metal is divided so that the main induced conduction 

 currents cannot flow, but only residual minor currents, we de- 

 stroy the diffusion effect more or less, according to the fineness 

 of the division, and leave only the inductivity effect. In my 



