Thus far in this chapter only those receivers
Thus far in this chapter only those receivers which are commonly
called _hand receivers_ have been discussed. These are the receivers
that are ordinarily employed by the general public.
[Illustration: Fig. 57. Operators Receiver]
Operators Receiver. At the central office in telephone exchanges
the operators are provided with receivers in order that they may
communicate with the subscribers or with other operators. In order
that they may have both of their hands free to set up and take down
the connections and to perform all of the switching operations
required, a special form of receiver is employed for this purpose,
which is worn as a part of a head-gear and is commonly termed a _head
receiver_. These are necessarily of very light construction, in
order not to be burdensome to the operators, and obviously they
must be efficient. They are ordinarily held in place at the ear by a
metallic head band fitting over the head of the operator.
[Illustration: GRANT AVENUE OFFICE OF 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. Operators 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 Galvanis 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 charge
with respect to the copper. When the two plates are connected by a
wire, a current flows from the copper to the zinc plate through the
metallic path of the wire, just as is to be expected when any
conductor of relatively high electrical potential is joined to one of
relatively low electrical potential. Ordinarily, when one charged body
is connected to another of different potential, the resulting current
is of but momentary duration, due to the redistribution of the charges
and consequent equalization of potential. In the case of the simple
cell, however, the current is continuous, showing that some action is
maintaining the charges on the two plates and therefore maintaining
the difference of potential between them. The energy of this current
is derived from the chemical action of the acid on the zinc. The cell
is in reality a sort of a zinc-burning furnace.