as the battery and the
parts contained in the generator and bell box.

[Illustration: Fig. 150. Circuit of Bridging Magneto Desk Set]

In Fig. 150 is shown the circuit of the Stromberg-Carlson magneto
desk-telephone set, illustrated in Fig. 145. This diagram needs no
explanation in view of what has already been said. The conductors,
leading from the desk-stand group of apparatus to the bell-box group of
apparatus, are grouped together in a flexible cord, as shown in Fig.
145, and are connected respectively to the various binding posts or
contact points within the desk stand at one end and at the base of the
bell box at the other end. These flexible conductors are insulated
individually and covered by a common braided covering. They usually are
individualized by having a colored thread woven into their insulating
braid, so that it is an easy matter to identify the two ends of the
same conductor at either end of the flexible cord or cable.

[Illustration: Fig. 151. Common-Battery Wall Set]

[Illustration: Fig. 152. Common-Battery Wall Set]

Common-Battery Telephone Sets. Owing to the fact that common-battery
telephones contain no sources of current, they are usually somewhat
simpler than the magneto type. The component parts of a
common-battery telephone, whether of the wall or desk type, are the
transmitter, receiver, hook switch, polarized bell, condenser, and
sometimes an induction coil. The purpose of the condenser is to
prevent direct or steady currents from passing through the windings of
the ringer while the ringer is connected across the circuit of the
line during the time when the telephone is not in use. The
requirements of common-battery signaling demand that the ringer shall
be connected with the line so as to be receptive of a call at any time
while the telephone is not in use. The requirements also demand that
no conducting path shall normally exist between the two sides of the
line. These two apparently contradictory requirements are met by
placing a condenser in series with the ringer so that the ringer will
be in a path that will readily transmit the alternating ringing
currents sent out from the central-office generator, while at the same
time the condenser will afford a complete bar to the passage of steady
currents. Sometimes the condenser is also used as a portion of the
talking apparatus, as will be pointed out.

[Illustration: MAIN OFFICE, KANSAS CITY HOME TELEPHONE CO., KANSAS
CITY, MO.]