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practicable only over distances of a few hundred feet. .
Carbon Transmitters Without Induction Coils can be used in series party-line systems as shown in Fig. 103. The same lettering is employed as in Fig. 102 for corresponding parts, so the different apparatus will be easily recognized. It will be noted an extra battery x serves in the two end stations for operating the transmitters. This system like the preceding one is practicable only over short distances. It is convenient in that it does not require the use of induction coils and magneto generators.
Ringing Current Supplied by Battery which furnishes the talking current, instead of by a magneto generator, is a feature of some series party-line systems. The wiring for a three-party-line system of this kind is shown in Fig. 104. Each station is STATION 1.
Fig. 103.-Series Party-Line System Using Carbon
Transmitters without Induction Coils
wired the same, and when not in use the receiver hook h is down and the switch s is up, leaving the battery bells in circuit. In systems where battery bells are used in series, of which the present one is an example, the vibrations must be short-cir
cuited in all bells but one. When a signal is to be sent, the switch s is pressed down, the receiver being left on the hook, and the battery at that station causes all the bells to ring. Upon removing the receiver from the hook switch, the latter flies up, connecting the same battery in series with
STATION 3 Fig. 104.-Series Party-Line System in which Both Ringing Current and Talking Current are Obtained from One Battery at Each Station
the transmitter t and primary winding p of the induction coil as in Station 3. The secondary winding c of the induction coil and the receiver r are then in connection with the line as shown, while the switch s and bell b are disconnected. It is obvious that in this system the talking current must flow through the bells of those stations not in use in order to reach its destination. The batteries should be connected as indicated, so that in case two or more signaling switches are depressed simultaneously, the batteries will agree in polarity.
Bridging Party-Line Systems.—These systems when wired other than according to the regular Bridging Connections previously given are arranged for selective signaling. By “selective signaling ” is meant that the bell in any one station of a party
line system can be rung without ringing the others. Bridging party-line systems thus fitted are generally connected to a central office or exchange where a switchboard and operator are provided to connect the station of the party signaling with the station of the party desired; in other words, systems of this kind are used where the number of stations and their importance are such as to warrant a comparatively high grade of service.
A Two-Party-Line Selective System wired on the bridging plan is shown in Fig. 105. Suppose a party on some other circuit entering the exchange has signaled the operator that he desires a connection with Station i shown in the diagram. The operator first pushes her master-key to the left so that it makes contact with the left-hand stop. When the plug is inserted in the jack connected to the desired line and the ringing key is closed, current from the magneto generator will pass out on the “sleeve" side of the line down through binding post 5 in Station 1, through its magneto bell, and by binding post G to ground and back to the grounded generator in the exchange. This current will ring the bell in Station i because this bell is connected between 5 and G; it will not, however, ring the bell in Station 2 although the bell there is similarly connected between 5 and G, because the binding post 5 is joined to the “ tip” side of the line instead of the “sleeve" side.
If the operator be requested to signal Station
2 she would push the master-key to the righthand contact and proceed as before, whereupon the ringing current would pass out on the “tip” side of the line and ring the bell in Station 2. The bell in Station I would not be affected for the reason previously given. Having raised the desired party the operator would connect the switchboard terminals of his line circuit with those of the calling party, thereby enabling them to converse with each other. By means of a lowresistance magnet drop connected across the line at the switchboard and cut out of circuit when the plug is inserted in the jack, either of the parties at Stations 1 or 2 can signal the operator with the bridging magneto generator in their telephone set, without disturbing the other. The bells at the stations are wound to a resistance of 1,000 ohms.
The principle of the selective system just described can advantageously be employed when there are any number of parties connected to a line. By having one-half of them connected with their bells between the “tip” side of the line and ground, and the remainder with their bells between the “sleeve” side of the line and ground, only half of the parties connected, instead of all of them, will be disturbed when signaled.
A Four-Party-Line Selective System wired on the bridging plan is shown in Fig. 106. This is a further development of the system in Fig. 105, employing positive and negative pulsating current to actų