of the cell or
battery the long thin line is supposed to represent the positively
charged plate and the short thick line the negatively charged plate.
The number of pairs may indicate the number of cells in the battery.
Frequently, however, a few pairs of such lines are employed merely for
the purpose of indicating a battery without regard to its polarity or
its number of cells.

[Illustration: Fig. 67. Battery Symbols]

In Fig. 67 the representation at _A_ is that of a battery of a number
of cells connected in parallel; that at _B_ of a battery with the
cells connected in series; and that at _C_ of a battery with one of
its poles grounded.

CHAPTER VIII

MAGNETO SIGNALING APPARATUS

Method of Signaling. The ordinary apparatus, by which speech is
received telephonically, is not capable of making sufficiently loud
sounds to attract the attention of people at a distance from the
instrument. For this reason it is necessary to employ auxiliary
apparatus for the purpose of signaling between stations. In central
offices where an attendant is always on hand, the sense of sight is
usually appealed to by the use of signals which give a visual
indication, but in the case of telephone instruments for use by the
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 mod