Conventional Symbol.  The conventional symbol for a cell, either of the primary or the secondary type, consists of a long thin line and a short heavy line side by side and parallel.  A battery is represented by a number of pairs of such lines, as in Fig. 67.  The two lines of each pair are supposed to represent the two electrodes of a cell.  Where any significance is to be placed on the polarity 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 modified forms it may be used with pulsating currents, that is, with periodically recurring impulses of current always in the same direction.