_Wall._ In Figs. 151 and 152 are given two views of a characteristic
form of common-battery wall-telephone set, made by the
Stromberg-Carlson Manufacturing Company. The common-battery wall set
has usually taken this general form. In it the transmitter is mounted
on an adjustable arm at the top of the backboard, while the box
containing the bell and all working parts of the instrument is placed
below the transmitter, the top of the box affording a shelf for
writing purposes. In Fig. 151 are shown the hook switch and the
receiver; just below these may be seen the magnets of the polarized
bell, back of which is shown a rectangular box containing the
condenser. Immediately in front of the ringer magnets is the induction
coil.

[Illustration: Fig. 153. Stromberg-Carlson Common-Battery Wall Set]

In Fig. 153 are shown the details of the circuit of this instrument.
This figure also includes a simplified circuit arrangement from which
the principles involved may be more readily understood. It is seen
that the primary of the induction coil and the transmitter are
included in series across the line. The secondary of the induction
coil, in series with the receiver, is connected also across the line
in series with a condenser and the transmitter.

_Hotel._ Sometimes, in order to economize space, the shelf of
common-battery wall sets is omitted and the entire apparatus mounted
in a small rectangular box, the front of which carries the transmitter
mounted on the short arm or on no arm at all. Such instruments are
commonly termed hotel sets, because of the fact that their use was
first confined largely to the rooms in hotels. Later, however, these
instruments have become very popular in general use, particularly in
residences. Sometimes the boxes or cabinets of these sets are made of
wood, but of recent years the tendency has been growing to make them
of pressed steel. The steel box is usually finished in black enamel,
baked on, the color being sometimes varied to match the color of the
surrounding woodwork. In Figs. 154 and 155 are shown two views of a
common-battery hotel set manufactured by the Dean Electric Company.

Such sets are extremely neat in appearance and have the advantage of
taking up little room on the wall and the commercial advantage of
being light and compact for shipping purposes. A possible disadvantage
of this type of instrument is the somewhat crowded condition which
necessarily follows from the placing of a