kind is shown in Fig. 15, in which pressure upon the key causes the
current from the battery to energize the sounder and give one sharp
audible rap of the lever upon the striking post.

_Vibrating Bell_. The vibrating bell, so widely used as a door bell,
is a device consequent to the telegraph. Its action is to give a
series of blows on its gong when its key or push button closes the
battery circuit. At the risk of describing a trite though not trivial
thing, it may be said that when the contact _1_ of Fig. 16 is closed,
current from the battery energizes the armature _2_, causing the
latter to strike a blow on the gong and to break the line circuit as
well, by opening the contact back of the armature. So de-energized,
the armature falls back and the cycle is repeated until the button
contact is released. A comparison of this action with that of the
polarized ringer (to be described later) will be found of interest.

[Illustration: Fig. 17. Elemental Magneto-Generator]

_Magneto-Bell._ The magneto-bell came into wide use with the spread of
telephone service. Its two fundamental parts are an
alternating-current generator and a polarized bell-ringing device.
Each had its counterpart long before the invention of the telephone,
though made familiar by the latter. The alternating-current generator
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.