THE ELECTRIC LIGHT. 675 



its electro-motive force to that of all the others; and 

 though it also adds its resistance, thereby diminishing the 

 quantity of current contributed by each convolution, the 

 integrated current becomes endowed with the power of 

 leaping across the successive spaces necessary for the pro- 

 duction of a series of lights in its course. The current is, 

 as it were, rendered at once thinner and more piercing by 

 the simultaneous addition of internal resistance and electro- 

 motive power. The machines, on the other hand, which 

 produce only a single light have a small internal resistance 

 associated with a small electro-motive force. In such 

 machines the wire of the rotating armature is compara- 

 tively short and thick, copper riband instead of wire being 

 commonly employed. Such machines deliver a large 

 quantity of electricity of low tension in other words, of 

 low leaping power. Hence, though competent when their 

 power is converged upon a single interval, to produce one 

 splendid light, their currents are unable to force a passage 

 when the number of intervals is increased. Thus, by 

 augmenting the convolutions of our machines we sacrifice 

 quantity and gain electro-motive force; while by lessening 

 the number of the convolutions we sacrifice electro- 

 motive force and gain quantity. Whether we ought to 

 choose the one form of machine or the other depends 

 entirely upon the external work the machine has to per- 

 form. If the object be to obtain a single light of great 

 splendor, machines of low resistance and large quantity 

 must be employed. If we want to obtain in the same cir- 

 cuit several lights of moderate intensity, machines of high 

 internal resistance and of correspondingly high electro- 

 motive power must be invoked. 



When a coil of covered wire surrounds a bar of iron, the 

 two ends of the coil being connected together, every 

 alteration of the magnetism of the bar is accompanied by 

 the development of an induced current in the coil. The 

 current is only excited during the period of magnetic 

 change. No matter how strong or how weak the mag- 

 netism of the bar may be, as long as its condition remains 

 permanent no current is developed. Conceive, then, the 

 pole of a magnet placed near one end of the bar to be 

 moved along it toward the other end. During the time of 

 the pole's motion there will be an incessant change in the 

 magnetism of the bar, and accompanying this change we 



