s ago (1831) the great Michael Faraday discovered that if a
loop of wire were moved up and down between the poles of an
electro-magnet (Fig. 66) a current was induced in the loop, its
direction depending upon that in which the loop was moved. The energy
required to cut the lines of force passed in some mysterious way into
the wire. Why this is so we cannot say, but, taking advantage of the
fact, electricians have gradually developed the enormous machines which
now send vehicles spinning over metal tracks, light our streets and
houses, and supply energy to innumerable factories.

[Illustration: FIG. 66.]

The strength of the current induced in a circuit cutting the lines of
force of a magnet is called its pressure, voltage, or electro-motive
force (expressed shortly E.M.F.). It may be compared with the
pounds-to-the-square-inch of steam. In order to produce an E.M.F. of one
volt it is calculated that 100,000,000 lines of force must be cut every
second.

The voltage depends on three things:--(1.) The _strength_ of the magnet:
the stronger it is, the greater the number of lines of force coming from
it. (2.) The _length_ of the conductor cutting the lines of force: the
longer it is, the more lines it will cut. (3.) The _speed_ at which the
conductor moves: the faster it travels, the more lines it will cut in a
given time. It follows that a powerful dynamo, or mechanical producer of
current, must have strong magnets and a long conductor; and the latter
must be moved at a high speed across the lines of force.

A SIMPLE DYNAMO.

In Fig. 67 we have the simplest possible form of dynamo--a single turn
of wire, _w x y z_, mounted on a spindle, and having one end attached to
an insulated ring C, the other to an insulated ring C^1. Two small
brushes, B B^1, of wire gauze or carbon, rubbing continuously against
these collecting rings, connect them with a wire which completes the
circuit. The armature, as the revolving coil is called, is mounted
between the poles of a magnet, where the lines of force are thickest.
These lines are _supposed_ to stream from the N. to the S. pole.

In Fig. 67 the armature has reached a position in which _y z_ and _w x_
are cutting no, or very few, lines of force, as they move practically
parallel to the lines. This is called the _zero_ position.

[Illustration: FIG. 67.]

[Illustration: FIG. 68.]

In Fig. 68 the armature, moving at right angles to the lines of force,
cuts a maximum number in