[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.]

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.