[Illustration:  Fig. 160.  Kellogg Common-Battery Desk Set]

[Illustration:  Fig. 161.  Dean Common-Battery Set]

Circuits of Common-Battery Telephone Sets.  The complete circuits of the Kellogg desk-stand arrangement are shown in Fig. 160, the desk-stand parts being shown at the left and the bell-box parts at the right.  As is seen, but two conductors extend from the former to the latter.  A simplified theoretical sketch is also shown in the upper right-hand corner of this figure.

The details of the common-battery telephone circuits of the Dean Electric Company are shown in Fig. 161.  This involves the use of the balanced Wheatstone bridge.  The only other thing about this circuit that needs description, in view of what has previously been said about it, is that the polarized bell is placed in series with a condenser so that the two sides of the circuit may be insulated from each other while the telephone is not in use, and yet permit the passage of ringing current through the bell.

[Illustration:  Fig. 162.  Monarch Common-Battery Wall Set]

The use of the so-called direct-current receiver has brought about a great simplification in the common-battery telephone circuits of several of the manufacturing companies.  By this use the transmitter and the receiver are placed in series across the line, this path being normally opened by the hook-switch contacts.  The polarized bell and condenser are placed in another bridge path across the line, this path not being affected by the hook-switch contacts.  All that there is to such a complete common-battery telephone set, therefore, is a receiver, transmitter, hook switch, bell, condenser, and cabinet, or other support.

The extreme simplicity of the circuits of such a set is illustrated in Fig. 162, which shows how the Monarch Telephone Manufacturing Company connect up the various parts of their telephone set, using the direct-current receiver already described in connection with Fig. 54.

[Illustration:  VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING]

CHAPTER XV

NON-SELECTIVE PARTY-LINE SYSTEMS

A party line is a line that is for the joint use of several stations.  It is, therefore, a line that connects a central office with two or more subscribers’ stations, or where no central office is involved, a line that connects three or more isolated stations with each other.  The distinguishing feature of a party line, therefore, is that it serves more than two stations, counting the central office, if there is one, as a station.

Strictly speaking, the term party line should be used in contradistinction to the term private line.  Companies operating telephone exchanges, however, frequently lease their wires to individuals for private use, with no central-office switchboard connections, and such lines are, by common usage, referred to as “private lines.”  Such lines may be used to connect two or more isolated stations.  A private line, in the parlance of telephone exchange working, may, therefore, be a party line, as inconsistent as this may seem.