1886.] On the Self-induction of an Electric Current. 467 



460, and a feeble increased resistance in the variable period of, but 

 10 per cent. ; if we now introduce a core of solid copper of 1*75 cm. 

 diameter and 5 cm. long, a great change takes place both in the force 

 of the extra currents and the resistance in the variable period, the 

 extra currents fall in value from 460 to 352, whilst the extra resis- 

 tance is increased to 52 per cent, above that of the stable period ; we 

 have here a double and contrary effect, the reduction of the force of 

 the extra currents and the increase in the resistance cannot be due to 

 the magnetic nature of copper, but must be due to the induced or so- 

 called "Foucault currents" circulating in the core; to prove this the 

 core was replaced by another similar in every respect but cut longi- 

 tudinally to its centre, the currents now ceased to circulate, and the 

 copper core had not the slightest effect either on the extra currents 

 or the variable resistance ; this is shown in the table, where for 

 greater precautions a core of insulated copper wires replaced the 

 solid core. Evidently the induced current in the core was the cause 

 of the extra resistance ; work was done by the primary current, and 

 a loss of energy at the expense of the electromotive force of the extra 

 currents, but in doing this work a resistance was produced which was 

 no doubt caused by the currents circulating in the core. These 

 currents required time, passing through the variable stage and thus 

 producing from their electromagnetic inertia an equivalent reaction 

 and electromagnetic inertia in the primary coil itself. That this 

 inertia is due to the electromagnetic character of the current and not 

 to an electric current considered apart, is proved by the fact (which I 

 have verified) that when we coil a wire into a coil of several super- 

 posed layers, its electromagnetic reactions introduce a measurable 

 resistance in the variable period precisely similar though feebler than 

 that which would be produced by the reaction on the conductor of a 

 magnetic body such as iron. 



The effect of a solid iron core and a bundle of iron wires on the 

 increase of the force of the extra currents is well known, but the 

 table shows an interesting result as to their effects on the variable 

 resistance; the solid iron core shows a very high force of extra 

 currents produced by its magnetic reaction on the wires of the helix, 

 the resistance in the variable period has increased 275 per cent., and 

 its extra currents are extremely high, as we should expect ; now if the 

 extra resistance is due to magnetic reaction alone it should increase 

 when we are enabled to increase this reaction, but if it is due in 

 greater part to the electromagnetic inertia of electric currents circu- 

 lating in the bar, then by preventing these currents from being formed 

 (as we did in the case of copper) we should greatly reduce the extia 

 resistance. This proved to be the case, for on replacing the solid iron 

 core by a bundle of fine iron wires the force of the extra currents rose 

 from 2338 to 5360, or more than double the force of that produced by 



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