Patent GB-190622113-A (GB)
Title: Improvements in Party Line Telephone Systems.
Filed: 1906-10-06
Granted: 1907-10-03
Abstract: 22,113. Select Telephone Manufacturing Co., and Bruce, W. M. Oct. 6. Telephone systems.-Relates to lock-out arrangements in central-battery party-line telephone systems, wherein the subscriber using the line cannot be disturbed by any other subscriber on the same line, but is provided with means for allowing the exchange operator to call a second subscriber on the same line. In the system shown in Fig. 2, the lock-out switch or relay 14 is included in a circuit bridging the line-wires 4, 5 but normally broken at the hookswitch contact 29. The microphone 17 and primary of the induction coil 18 are also included in a circuit bridging the line-wires, but normally broken at the hook-switch contact and at the armature contact 21 of the relay 14. When the receiver 10 is removed from the hook switch, the relay is energized by current passing from the line 5, through the wire 7, contact 29, relay 14, circuitcloser 16, wire 13, to the line 4 and exchange battery. The relay 14 then closes contact 21, completing the microphone circuit from the line 5 through the wires 7, contact 29, wire 19, key 20, armature contact 21, primary, microphone 17, to the line 4. The resistance of these two bridging- circuits is such as to utilize all the line current to such an extent that no other relay will be operated, and consequently no other subscriber can be connected with the exchange until the first subscriber replaces his receiver. In order to prevent possible actuation of a second relay by physical jarring of the relay instrument, the gravity circuit-closer 16, consisting of a cylindrical rod in suitable bearings, is arranged in the relay circuit and mounted in the casing enclosing the relay. Any jarring of the instrument causes the circuit-breaker to act first and prevent excitation of the relay. In order to prevent actuation of a second relay by the increase in the line current due to great decrease in the microphone resistance of the first subscriber, the effect of this variation of resistance is limited by shunting the microphone by the resistance 25. When it is desired to connect in a second subscriber on the same line, the first subscriber depresses the key 20, thus breaking the bridging- circuit through his telephone instruments, so that the line current will be sufficiently powerful to energize the relay 14 of the second subscriber, when that subscriber removes his receiver in response to an exchange call. Having once attracted the armature of the relay, the current is able to keep the relay energized even when the first subscriber releases the key 20. The depression of the key 20 throws the condenser 23 in circuit with the microphone, so that the first subscriber can overhear conversation between the operator and the second subscriber, although the direct-current shunt through the microphone is broken. The first subscriber thus learns when the desired connexion is established and then releases the key 20. Fig. 1 shows a modified construction of the relay, applicable in systems in which the stations are at long and varying distances from the exchange. The relay is provided with two energizing - windings 14, 15 in series, the coil 14 being short-circuited on the attraction of the armature 21. The condenser 23 in the system shown in Fig. 2, is replaced by a winding 27 inductively associated with the relay coils, so that on the depression of the key 20, the subscriber's telephone instruments are included in the local circuit of the winding 27, and are thus inductively connected to the line-wires for the same purpose as before. Harmonic or other selective calling arrangements may be employed.