Fig. 355. Dean Party Line Ringing Key
[View full size illustration.]

Fig. 356. Dean Party Line Ringing Key
[View full size illustration.]

Stromberg-Carlson Multiple Board. Line Circuit. In Fig. 357 is shown the multiple common-battery switchboard circuits employed by the Stromberg-Carlson Telephone Manufacturing Company. The subscriber's line circuits shown in this drawing are of the three-wire type and, with the exception of the subscriber's station, are the same as already described for the Western Electric Company's system.

Cord Circuit. The cord circuit employed is of the two-conductor type, the plugs being so constructed as to connect the ring and thimble contacts of the jack when inserted. This cord circuit is somewhat similar to that employed by the Kellogg Switchboard and Supply Company, shown in Fig. 352, except that only one battery is employed, and that certain functions of this circuit are performed mechanically by the inter-action of the armatures of the relays.

Supervisory Signals. When the answering plug is inserted in a jack, in response to a call, the current passing to the subscriber's station and also through the cut-off relay must flow through the relay 1, thus energizing it. As the calling subscriber's receiver is at this time removed from the hook switch, the path for current will be completed through the tip of the jack, thence through the tip of the plug, through relay 2 to ground, causing relay 2 to be operated and to break the circuit of the answering supervisory lamp. The two relays 1 and 2 are so associated mechanically that the armature of 1 controls the armature of 2 in such a manner as to normally hold the circuit of the answering supervisory lamp open. But, however, when the plug is inserted in a jack, relay 1 is operated and allows the operation of relay 2 to be controlled by the hook switch at the subscriber's station. The supervisory relay 3 associated with the calling cord is operated when the calling plug is placed in a jack, and this relay normally holds the armature of relay 4 in an operated position in a similar manner as the armature of relay 1 controlled that of relay 2. Supervisory relay 4 is under the control of the hook switch at the called subscriber's station.

Test. In this circuit, as in several previously described, when a plug is inserted in a jack of a line, the thimble contacts of the jacks associated with that line are raised to a higher potential than that which they normally have. The operator in testing a busy line, of course having previously moved the listening key to the listening position, closes a path from the test thimble of the jack, through the tip of the calling plug, through the contacts of the relay 4, the inside springs of the listening key, thence through a winding of the induction coil associated with her set to ground. The circuit thus established allows current to flow from the test thimble of the jack through the winding of her induction coil to ground, causing a click in her telephone receiver. The arrangement of the ringing circuit does not differ materially from that already described for other systems and, therefore, needs no further explanation.

Fig. 357. Stromberg-Carlson Multiple Board Circuits
[View full size illustration.]

Multiple Switchboard Apparatus. Coming now to a discussion of the details of apparatus employed in multiple switchboards, it may be stated that much of the apparatus used in the simpler types is capable of doing duty in multiple switchboards, although, of course, modification in detail is often necessary to make the apparatus fit the particular demands of the system in which it is to be used.