In the last chapter was described the subscribers' drop and jack wiring. An illustration is here shown of another form of key for the cord circuit which has decided advantages over the one previously mentioned, in-as-much as the operator is enabled to answer a call with one cord of a pair, make connection with the party desired with one movment of the key, and listen in on the line when desired.

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The key here shown is constructed of spring-brass strips and silver contacts. The circular plunger between the strips is actuated by a pivotal handle, so that the operator may at will force the plunger between the strips, thereby opening off one circuit and connecting with another. The plugs are what is commonly described as two part plugs and are connected to the key springs exactly as shown in the drawing. When the drop falls, thereby signalling the operator, the plug No. 1 is inserted and conversation established by pressing the cam lever so that the plunger is forced between the No. 1 springs and away from the springs of No. 2 for the time being, placing the transmitter springs in contact. When party No. 1 is to be connected with the party called, the plug No. 2 is inserted in the desired jack and that party signalled by again moving the ringing key, but this time in the opposite direction, and ringing on the line by whatever device may have been provided for the purpose. Then the plunger is released and when in the normal position party No. 1 is in communication with party No. 2. The operator may at any time throw her set in upon the line and enter into conversation.

The operator's set consists of the primary side of a telephone induction coil in series with battery and a transmitter. The receiver circuit comprises the scondary of the induction coil, closed through a telephone receiver. It must be remembered, however, that there are many styles of keys used in connection with cords of a switch board, each possessing some particular fitness for the circuit involved. There is one drawback to the one here described, as the operator can ring on only one cord of a pair and, the receiver being in the secondary circuit, there will be a variance in the hearing qualities, all of which can be perfected according to the ingenuity of the amateur electrician, who will find it greatly to his advantage to construct one of these circuits in his workshop.

Often times it is not desirable to place the transmitter set directly upon the line, and recourse is taken to a modified form of the induction coil, in which the transmitter, battery and primary winding constitute one side of the set and the secondary winding, in series with the receiver, becomes the side which is placed directly upon the cord circuit when the key springs are actuated by the plunger. A condenser of low capacity might also be connected in parallel with the receiver in the secondary and benefit transmission.

The induction coils in common use in transmission are constructed of No. 23 D. C. C. wire wound ill turns about an iron core to a resistance of about one and one-half ohms. The secondary is of much finer wire, preferably No. 36, wound in turns over the primary, to a resistance of at least 150 ohms. Such a coil is adaptable to long distance transmission where the potential raising value of the secondary is sufficient to overcome the resistance of the line and equipment. For strictly local usage it may be desirable to use No. 28 wire in the secondary and wind to a resistance of 10 ohms in the primary and 20 in the secondary, the exact efficiency being found by experiment.

In the next chapter will be described a few circuits pertaining to central energy work/where all the signalling and talking battery is derived from a central supply instead of batteries and generators being required at each telephone.