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 
