erator
of the magneto-bell consists of a rotatable armature composed of a
coil of insulated wire and usually a core of soft iron, its rotation
taking place in a magnetic field. This field is usually provided by a
permanent magnet, hence the name "magneto-generator." The purist in
terms may well say, however, that every form whatever of the
dynamo-electric generator is a magneto-generator, as magnetism is one
link in every such conversion of mechanical power into electricity.
The terms magneto-electric, magneto-generator, etc., involving the
term "magneto," have come to imply the presence of _permanently_
magnetized steel as an element of the construction.

In its early form, the magneto-generator consisted of the arrangement
shown in Fig. 17, wherein a permanent magnet can rotate on an axis
before an electromagnet having soft iron cores and a winding.
Reversals of magnetism produce current in alternately reversing
half-cycles, one complete rotation of the magnet producing one such
cycle. Obviously the result would be the same if the magnet were
stationary and the coils should rotate, which is the construction of
more modern devices. The turning of the crank of a magneto-bell
rotates the armature in the magnetic field by some form of gearing at
a rate usually of the order of twenty turns per second, producing an
alternating current of that frequency. This current is caused by an
effective electromotive force which may be as great as 100 volts,
produced immediately by the energy of the user. In an equipment using
a magneto-telephone as both receiver and transmitter and a
magneto-bell as its signal-sending machine, as was usual in 1877, it
is interesting to note that the entire motive power for signals and
speech transmission was supplied by the muscular tissues of the
user--a case of working one's passage.

[Illustration: Fig. 18. Extension of a Permanent Magnet]

The alternating current from the generator is received and converted
into sound by means of the _polarized ringer_, a device which is
interesting as depending upon several of the electrical, mechanical,
and magnetic actions which are the foundations of telephone
engineering.

[Illustration: Fig. 19. Extension of a Permanent Magnet]

"Why the ringer rings" may be gathered from a study of Figs. 18 to 21.
A permanent magnet will impart temporary magnetism to pieces of iron
near it. In Fig. 18 two pieces of iron are so energized. The ends of
these pieces whic