[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 all the parts in so confined a space.  This interferes somewhat with the accessibility of the various parts, but great ingenuity has been manifested in making the parts readily get-at-able in case of necessity for repairs or alterations.

[Illustration:  Fig. 154.  Steel Box Hotel]

[Illustration:  Fig. 155.  Steel Box Hotel Set]

Desk.  The common-battery desk telephone presents a somewhat simpler problem than the magneto desk telephone for the reason that the generator and local battery, the two most bulky parts of a magneto telephone, do not have to be provided for.  Some companies, in manufacturing desk stands for common-battery purposes, mount the condenser and the induction coil or impedance coil, or whatever device is used in connection with the talking circuit, in the base of the desk stand itself, and mount the polarized ringer and the condenser used for ringing purposes in a separate bell box adapted to be mounted on the wall or some portion of the desk.  Other companies mount only the transmitter, receiver, and hook switch on the desk stand proper and put the condenser or induction coil, or other device associated with the talking circuit, in the bell box.  There is little to choose between the two general practices.  The number of conducting strands in the flexible cord is somewhat dependent on the arrangement of the circuit employed.