Patent GB-305730-A (GB)
Abstract: 305,730. Graham & Co., Ltd., A., and Rickets, W. J. Nov. 16, 1927. Direct call and manual exchange systems.-In two-wire systems with central-battery working, each substation is provided with a rectifier of the dry type consisting of metal and metallic oxide in contact, the rectifier being associated with a relay which controls the circuit of a direct current bell. According to one arrangement, the rectifier and relay are in parallel with one another and in series with a condenser across the line, and a. ringing signal is transmitted by pulsating direct current. In a second arrangement, the rectifier and relay are normally in series with one another across the line and a ringing signal is transmitted by a single reversal of current over the line conductors. Each arrangement is described as applied to both direct-call and manual-exchange systems. First arrangement. When a calling subscriber X, Fig. 2, removes his receiver, microphone 10 and receiver 12 are bridged across lines 3, 4 completing a circuit for line relay 5 which lights the calling subscriber's lamp 14 and operates a common signal 9<e>. The exchange operator inserts one plug 20 of the single-conductor cord 21 in jack 15 and the other plugs 20<a> in jack 15<b> leading to her ringing and speaking apparatus, and operates key 22 to close contacts 22b, thus completing a talking circuit. She then transfers plug 20 to the wanted subscriber's jack 15<a> and operates key 22 in the opposite direction to connect up a buzzer 2<e> which causes pulsating current to flow over jack 15<a>, line 3<a>, condenser 6a, rectifier 7<a> and relay 8<a> in parallel, and back over line 4<a>. The rectifier causes unidirectional current to flow in relay 8<a> which operates and connects bell 9<a> directly across leads 3<a>, 4a. When the called subscriber replies relay 5<a> and lamp 14<a >operate and the operator transfers plug 20<a> from jack 15<b> to jack 15. The lamps 14, 14<a> remain lit until the end of conversation. The relay 8<a> may be made slow-acting to render it insensitive to the pulsations set up by the bell action. In directcall systems, pulsating current is generated at a substation by a buzzer which is connected across the line conductors by means of a key. Second arrangement. In a direct-call system, Fig. 3, each substation has.a relay 8 and rectifier 7 normally connected in series over the switch hook 11 across the line conductors 3, 4. The poles of the central battery are connected to the line conductors over impedance windings 24, 24<a> which oppositely control an armature 30 to operate pole-changing springs. The normal direction of current in the line conductors is not such as to pass the rectifiers, but when a subscriber X, say, lifts his receiver, the loop through his instruments causes winding 24 to operate the righthand springs, thereby reversing the direction of current over line conductors 3<a>, 4a. Rely 8<a> then operates and connects up the bell 9<a>. In a manual-exchange system, the reversal of current to initiate ringing is effected by a key associated with a two-conductor cord. The line relay (corresponding to relay 5. Fig. 2) has in series with it a resistance having a positive temperature coefficient in order to limit the current through the relay which, when the reversing key is operated, is otherwise directly across the battery.