icient electromotive force. This is done through
multiplying, by the use of thin wires, the convolutions of the
rotating armature as, a moment ago, we augmented the cells of our
voltaic battery. Each additional convolution, like each additional
cell, adds 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 production 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 comparatively 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 perform. If
the object be to obtain a single light of great splendour, machines of
low resistance and large quantity must be employed. If we want to
obtain in the same circuit 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 magnetism 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