HOME TELEPHONE COMPANY, SAN
FRANCISCO, CAL. A Type of Central-Office Buildings in Down-Town
Districts of Large Cities.]

Such a receiver is shown in cross-section in Fig. 57, and completely
assembled with its head band in Fig. 58. Referring to Fig. 57 the
shell _1_ of the receiver is of aluminum and the magnets are formed of
steel rings _2_, cross-magnetized so as to present a north pole on one
side of the ring and a south pole on the other. The two L-shaped pole
pieces _3_ are secured by screws to the poles of these ring magnets,
and these pole pieces carry the magnet coils, as is clearly indicated.
These poles are presented to a soft iron diaphragm in exactly the same
way as in the larger hand receivers, the diaphragm being clamped in
place by a hard rubber ear piece, as shown. The head bands are
frequently of steel covered with leather. They have assumed numerous
forms, but the general form shown in Fig. 58 is the one commonly
adopted.

[Illustration: Fig. 58. Operator's Receiver and Cord]

[Illustration: Fig. 59. Receiver Symbols]

Conventional Symbols. The usual diagrammatic symbols for hand and
head receivers are shown in Fig. 59. They are self-explanatory. The
symbol at the left in this figure, showing the general outline of the
receiver, is the one most commonly used where any sort of a receiver
is to be indicated in a circuit diagram, but where it becomes
desirable to indicate in the diagram the actual connections with the
coil or coils of the receiver, the symbol shown at the right is to be
preferred, and obviously it may be modified as to number of windings
and form of core as desired.

CHAPTER VII

PRIMARY CELLS

Galvani, an Italian physician, discovered, in 1786, that a current of
electricity could be produced by chemical action. In 1800, Volta, a
physicist, also an Italian, threw further light on Galvani's discovery
and produced what we know as the _voltaic_, or _galvanic_, cell. In
honor of these two discoverers we have the words volt, galvanic, and
the various words and terms derived therefrom.

Simple Voltaic Cell. A very simple voltaic cell may be made by
placing two plates, one of copper and one of zinc, in a glass vessel
partly filled with dilute sulphuric acid, as shown in Fig. 60. When
the two plates are not connected by a wire or other conductor,
experiment shows that the copper plate bears a positive charge with
respect to the zinc plate, and the zinc plate bears a negative ch