public, the sense of hearing is appealed to by employing an audible
rather than a visual signal.

Battery Bell. The ordinary vibrating or battery bell, such as is
employed for door bells, is sometimes, though not often, employed in
telephony. It derives its current from primary batteries or from any
direct-current source. The reason why they are not employed to a
greater extent in telephony is that telephone signals usually have to
be sent over lines of considerable length and the voltage that would
be required to furnish current to operate such bells over such lengths
of line is higher than would ordinarily be found in the batteries
commonly employed in telephone work. Besides this the make-and-break
contacts on which the, ordinary battery bell depends for its operation
are an objectionable feature from the standpoint of maintenance.

Magneto Bell. Fortunately, however, there has been developed a
simpler type of electric bell, which operates on smaller currents, and
which requires no make-and-break contacts whatever. This simpler form
of bell is commonly known as the _polarized_, or _magneto_, bell or
_ringer_. It requires for its operation, in its ordinary form, an
alternating current, though in its modified 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