[Illustration: FIG. 2.]
It has already been mentioned, that Oersted found that a magnet when
free to turn tended to set itself at right angles to a wire in which a
current of electricity was passing, thus demonstrating some inter-action
between electricity and magnetism; but it remained for Faraday to
discover the converse fact, namely, that a magnet moving across a wire,
the ends of which were connected with a galvanometer or otherwise
closed, originated a current of electricity in the wire, the direction
of which depended upon the direction of the movement of the magnet. If
the wire was coiled into a hollow helix, the magnet in moving through
the helix moved across, that is, at right angles to all the turns of
the helix; and each complete turn added to the intensity of the current.
This will be understood by reference to the diagram, Fig. 2. Let G be a
galvanometer connected with the wires from a helix; N S, a permanent bar
magnet. If the magnet be thrust into the coil, a current of electricity
will traverse the helix, wire, and galvanometer, and the needle will
indicate its direction. If the magnet be now withdrawn, a current will
move in the opposite direction through the whole circuit. The
electricity that is thus originated is said to be induced. The quantity
of electricity that can be induced thus is almost unlimited, depending
upon the size and strength of the magnet, the size of the wire, and the
length of wire in the coil. There are now many forms of machines for
developing electricity from the motion of coils of wire in front of the
poles of permanent magnets. They are generally called magneto-electric
machines. The action involved in these machines is so important in its
bearing upon telephony as to necessitate a fuller description of them.