Patent GB-189707774-A (GB)
Title: Improvements in Selective Signal Apparatus for Party Telephone Lines.
Inventor: KINGSBURY JOHN EDWARD
Filed: 1897-03-25
Granted: 1897-05-29
Abstract: 7774. Kingsbury, J. E., [Western Electric Co.]. March 25. Telephone systems. - An exchange system is described in which not more than twelve sub-stations are connected between two wires from the exchange ; any one can be called separately from the exchange. If not more than ten sub-stations are connected to each pair of wires, means can be provided for locking all the instruments at the substations except one called, so that interruption of communication with this one by the others is made impossible. The diagram shows the connections of one sub-station S between the wires A, B and an annunciator I at the exchange. Each sub-station has two relays R, R<2>, connecting the wires A, B to earth E<2>; these connections may vary at different sub-stations. At some of the sub-stations one relay, such as R', is neutral or compound, holding a local call circuit c normally closed, so that any current opens it. The other relays are polarized and hold the local circuits normally open, so that currents in proper directions close them. Each local circuit c is thus normally open at one or both relays. The pairs of relays differ or are connected differently at all the sub-stations, so that any pair can be operated by connecting the wires A, B and earth in different ways to the poles of a battery at the exchange. The connections are made by keys, a separate one making two or three connections being provided for each sub-station on the pair of wires. When the contacts of both relays are closed at any sub-station, a bell in the circuit c is rung by the local battery s. When in use, the telephone switch-lever L, by connections not shown, places this battery s in the local transmitter circuit. The exchange selecting-keys are connected with the wires A, B through a plug P. When this is removed the wires A, B are in communication through a differentiallywound annunciator I and a battery G<2> with earth, so that equal currents flow constantly through both wires A, B and through the relays R, R<2> to earth. When any switch-lever L is moved it temporarily short-circuits the relay R' through a spring d and operates the annunciator. For locking the telephones not in use, each sub-station is provided with a device R' in which a complete iron magnetic circuit is wound with two coils, one connecting each relay R, R' to earth. Two of the exchange keys are arranged to make both wires A, B either positive or negative to earth. When either of these keys is operated, consequent poles are produced at f, h on arms of the magnetic circuit, and a polarized armature j is moved towards one or the other pole, depending on which key has been operated. In one position, the armature j prevents movement of the telephone switchlever L and gives a visible signal that the wires are in use. The arms f, h are wound with a branch of the local circuit c, so that, if this is closed by the relays R, R', the armature j is moved to free the switch-lever and allow communication at any one sub-station. All the switch-levers are freed by operating the other key at the central station and are normally left so. The ten current-combinations used for operating the relays at the sub-stations do not magnetize the device R<3> so as to produce polarity at f, h.