ified forms it may be used with
pulsating currents, that is, with periodically recurring impulses of
current always in the same direction.

Magneto Generator. In the early days of telephony there was nearly
always associated with each polarized bell a magneto generator for
furnishing the proper kind of current to ring such bells. Each
telephone was therefore equipped, in addition to the transmitter and
receiver, with a signal-receiving device in the form of a polarized
bell, and with a current generator by which the user was enabled to
develop his own currents of suitable kind and voltage for ringing the
bells of other stations.

Considering the signaling apparatus of the telephones alone,
therefore, each telephone was equipped with a power plant for
generating currents used by that station in signaling other stations,
the prime mover being the muscles of the user applied to the turning
of a crank on the side of the instrument; and also with a
current-consuming device in the form of a polarized electromagnetic
bell adapted to receive the currents generated at other stations and
to convert a portion of their energy into audible signals.

The magneto generator is about the simplest type of dynamo-electric
machine, and it depends upon the same principles of operation as the
much larger generators, employed in electric-lighting and
street-railway power plants, for instance. Instead of developing the
necessary magnetic field by means of electromagnets, as in the case of
the ordinary dynamo, the field of the magneto generator is developed
by permanent magnets, usually of the horseshoe form. Hence the name
_magneto_.

[Illustration: Fig. 68. Principles of Magneto Generator]

In order to concentrate the magnetic field within the space in which
the armature revolves, pole pieces of iron are so arranged in
connection with the poles of the permanent magnet as to afford a
substantially cylindrical space in which the armature conductors may
revolve and through which practically all the magnetic lines of force
set up by the permanent magnets will pass. In Fig. 68 there is shown,
diagrammatically, a horseshoe magnet with such a pair of pole pieces,
between which a loop of wire is adapted to rotate. The magnet _1_ is
of hardened steel and permanently magnetized. The pole pieces are
shown at _2_ and _3_, each being of soft iron adapted to make good
magnetic contact on its flat side with the inner flat surface of the
bar magnet, a