628 



THE POPULAR SCIENCE MONTHLY. 



body, and is the same for all situations or conditions of the body, 

 we shall see that self-induction has but a very faint likeness to it, 



for self-induction is a property 

 of a conducting path or cir- 

 cuit, and not at all of the cur- 

 rent. To dip lightly into the 

 theory of the phenomenon, we 

 may say that the inception or 

 the stoppage of an electrical 

 flow in any conductor involves 

 the starting up or stoppage of 

 a movement in the dielectric 

 medium surrounding the con- 

 ductor. The time requisite for 

 this movement to start up or 

 stop gives a perfect analogue 

 to mechanical inertia. If, 

 now, we have a circuit consist- 

 ing of a wire returning on it- 

 self, the two halves being as 

 close together as they may be 

 without touching, we see that 

 a flow starting up in this wire 

 means a current in each half 

 in opposite directions. For the 

 present it suffices to say that 

 the effect above referred to of 

 the starting up of a movement 

 in the surrounding medium is 

 rendered less and less by the 

 canceling effect of the oppo- 

 site electrical flows the nearer 

 the two halves of the circuit 

 are brought together. 



The evil effects of self-in- 

 duction are directly propor- 

 tional to the number of alter- 

 nations of the current in a 

 given time, and consequently 

 the twenty-five-period current 

 adopted for the Niagara Falls 

 work is highly advantageous 

 from this point of view. 



The so-called "skin resist- 

 ance ' of an alternating-current circuit is, in brief, due to the fact 

 that an alternating current penetrates only a short distance into 



Wheel case. 



Fig. 8. Wheel Case, Shaft, and Dynamo. 



