Patent GB-190421819-A (GB)
Title: Improvements in Automatically Operated Telephonic Exchanges for a Small Number of Lines.
Inventor: BULLARD ALBERT MORRISON
Filed: 1904-10-11
Granted: 1904-12-15
Abstract: 21,819. Bullard, A. M. Oct. 11. Telephone systems; telephone calls.-Relates to automatic telephone exchange systems adapted more particularly for use by a small number of subscribers, some of whom may be on party lines. A central battery aud a central call generator are used, and each subscriber (or party line) is furnished with a switch at the exchange, the rotary arm of which can make contact with the terminals of any other line, unless such line is already engaged at another switch, A " reverting " call, that is, a call to a subscriber on the same party line, is made when the rotary switch arm makes contact with a pair of terminals connected to the line itself, since, for calling purposes, one of the contacts of the switch arm is momentarily disconnected from the corresponding side of the calling- subscriber's line, by means of the "ringing-relay," and is connected to the ringing-generator at the exchange. The talking circuit of two connected lines L, L<2> is shown in Fig. 7, B being the common battery and K the condensers which transmit the voice-currents. The relay R which controls the subscriber's switch and the ringing-relay R<2> are not in the talking circuit. Pulsating unidirectional current is used for operating the subscribers' call bells, and the ringing and selecting relays are polarized so as not to respond to the calling-currents. Fig. 1 shows two lines L, L<2> and their connections at the exchange. Both lines are party lines, three subscribers' stations S<3>, S<4>, S<5> being shown on the line L'. With each subscribers' line are associated three relays R, R', R<3> and three electromagnets M, M<2>, M<3>. The relay R controls the supply of current through the wire 38 to the electromagnets M, M<2>, M<3> of the subscriber's switch. The relay R<3> is the test relay which gives an "engaged" signal from the rotary interrupter P<2> to the calling- subscriber if an attempt is made to connect up to a line already in use, and at the same time disconnects the interfering subscriber's switch arm from the terminals of such a line as soon as it makes contact therewith. The switch comprises a vertical shaft s, Fig. 4, which carries the switch arm s', which can be moved round step by step, above and clear of the pairs of line contacts u, v, by means of the ratchet-wheel S<3>, pawl s<4>, and electromagnet M. The shaft is under spring control, and tends to fly back to the zero position. During its step-by-step or selective movement, the sluggishly - acting electromagnet M<2> keeps the detent pawl s<5> in engagement with the ratchetwheel, and, when the prolonged current which follows the intermittent selective currents establishes sufficient contact between the armature of the electromagnet M and its back stop t, current from the same wire 38 flows through the electromagnet M<3> and depresses the shaft s against the tension of the spring s<7> so as to engage the pair of contacts e, f on the arm s<2> with the contacts u, v of the selected line. While the magnet M<3> is energized, the circuit of the electromagnets M, M<2> is broken at the contact 39, but the switch arm is prevented from flying back to the zero position by its engagement with the contacts u, v, and this engagement persists until the electromagnet M<3> is de-energized. In Fig. 1, the contacts u, v are shown, for convenience, at different parts of the diagram, and are shown in multiple for each of the two lines concerned, to represent the multipling of each line on all the switches. The switch of the calling-line L has its contacts e, f engaged with contacts u, v of the line L<2>. The switch of the line L<2> is idle, its contacts e, f being disconnected, as shown. A. cam Q, Fig. 4, on the shaft s causes the switching over of certain contacts C as soon as the shaft leaves the zero position. The effect is to close the contacts 47, 49 and open the contacts 45, 46, thus connecting the multipled contacts v of the calling-line to the " engaged " signal generator P<2> and disengaging the contacts u from the line. The electromagnet M<2>, in addition to operating the detaining-pawl s<5>, operates contact-arms 57, 60, so that while it is energized contact is broken at 57, 58 and made at 60, 61. At each subscriber's station, an interrupting-device E for sending the selective caning-currents to line is provided in addition to the bell D, induction coil h, condenser K<2>, hook switch H, microphone T<2>, and ringing-key N. The interrupter may be of any suitable construction, and is conventionally represented as a star-wheel in the diagram. The operation of the system is as follows :-A subscriber on the line L, desiring to establish communication with the line L<2>, first lifts his receiver from the hook H and thus completes the metallic circuit of the line L. The controlling and ringing relays R, R<2> of that line are in consequence energized by the battery B. The interrupter E is next operated, the arm w<2> being turned clockwise through an angle depending on the number of the line to be called, and then released. The removal of the arm w<2> from contact with the insulating-block on the end of the contact-spring 34 effects the connection of the wire 31 to the line-wire 3, the opening at 29 of the ground connection of the ringing-key N, and the opening at 24 of the metallic circuit of the line L<2>. As the arm w<2> returns to its normal position, it makes intermittent contact at 33, thus putting the line 3 intermittently to earth and energizing the relay R intermittently. Intermittent currents consequently flow through the wire 38 and electromagnets M, M<2> to earth. The switch arm is thus spiced round, as already explained, until its contacts are immediately above those of the "wanted" line. At this time, as explained above, contact is made at 60, 61, so that the coil r<*> of the test relay R<3> is connected to the contact f of the switch arm. A prolonged current which follows the intermittent currents causes the energization of the electromagnet M<3>, as already explained, and engages the switch-arm contacts e, f with the contacts u, v of the " wanted line. The electromagnet M' remains energized throughout the conversation, that is, until the circuit of the relay R is broken. If this line is engaged, the test relay is energized and the connection is immediately broken. The arm w<2> of the interrupter E having now returned to the position shown in Fig. 1, the metallic loop of the line L is again complete, and the relays R<2>, R are both energized. When the calling-subscriber depresses his calling-key N, he breaks this circuit and puts the line 3 to earth. The relay R consequently remains energized, but the relay R<2> is de-energized. The contact thus made at 7 sends ringing-current from the generator P through the wire g and contact e to the called subscriber's line-wire 2, returning by the line-wire 3 and resistance r<2> (shunting the relay R) through the battery B to earth. If the called subscriber is on a party line, the key is depressed repeatedly in accordance with a prearranged code. An inductance coil p, which is short-circuited while the relay R<2> is energized, prevents loss of ringing-current through the relay R<2> of the called subscriber's line. When the called subscriber lifts his receiver from the hook in response to the call, he closes the metallic loep of his own line, all four relays R, R<2> being now energized, and the talking circuit established as shown in Fig. 7. The de-energization of the relays when the receivers are returned to the hooks at the end of the conversation is followed by the de-energization of the "connecting" electromagnet M<3> of the calling-subscriber's switch, and all the parts then return to the normal position. In the case of a " reverting call, the switch arm connects up to the calling-line itself, which consequently receives the ringing-current. The talking circuit is in this case a double bridge on the single metallic loop. G, G<2> denote earth connections in Fig. 1.