377,973. Automatic exchange systems. ASSOCIATED TELEPHONE & TELEGRAPH CO., 1033, West Van Buren Street, Chicago, U.S.A. April 30, 1931, No. 12799. Convention date, June 27, 1930. [Class 40 (iv).] Thermionic valves are used as the impulse repeating elements in switches and repeaters. When used in switches the responding magnets are connected directly to the plate of the valve and a slow relay, such as the change over relay, is also included in this circuit. Speci. fication 241,522, [Class 40 (iv), Telephones &c.], is referred to. When a call originates LS hunts for a free selector OS1 and 107 pulls up, earthing the guard conductor 105 and completing the valve filament circuit. The receipt of the first impulse train changes the valve grid potential and vertical magnet 113 responds to step wipers 118, 119, 120 to the required level. Slow relay 108, which operated in series with 113, releases at the end of the train and as 109 operated and locked when 108 energized, magnet 114 operates and interacts with 109 to step the wipers over the level. When a free circuit is reached 110 pulls up and switches through, opens the filament circuit and removes 107 from the line. Relay 131 in the outgoing repeater responds and 132 operates, earthing 129 and completing the filament circuit of 136. The next impulse trains are repeated by 133 to ThS1 and HS1 which are similar to OS1, and relay 134 shorts out 135 from the impulsing circuit during the trains. Selector HS1 extends the call to a connector such as C1 and 201 responds followed by 203 which completes the filament circuit for 211. The next impulse train is repeated by 211 to vertical magnet 218 whilst the final train energizes rotary magnet 219, the change over being effected by 204. Relay 207 energizes in parallel with 219 to connect 206 to the wanted line. If the wanted line is busy 206 operates and, when 207 falls away, locks to 234. The filament circuit is broken and busy tone from 251 is returned to the calling party. If the line is free 206 remains back and when 207 releases 208 pulls up and locks to 232, breaking the filament circuit earthing 221 and connecting ringing from 250 to the wanted line. When the called party replies 205 energizes and locks, switching the circuit through. Relay 202 energizes cutting resistances 223 and 209 from the calling loop and enabling 135 to pull up. Resistances 143 and 139 are removed from the circuit of 131 to provide sufficient current for speech purposes. Relay 135 can also be used for metering. When the called party clears 202 relapses and resistances 209, 223, 143 and 139 are replaced and when the calling party clears 133 energizes breaking the circuit 142 to release the selectors TAS1, HS1 and C1. 131 falls away followed by 132 and 133 and the removal of earth from 129 enables OS1 and LS1 to restore in the usual way. Systems in which the repeater precedes the first selector and repeats the impulses to switches of ordinary type, provided with line relays, in which the relay in the plate circuit remains energized during conversation to hold the operated switches, in which the plate circuit of the repeater valve includes the two line conductors in series, and in which reversed battery is used for metering and supervision are also described. The plate of the valves may be connected to the ordinary exchange battery or to a special battery and the grids may be either normally at earth or at negative potential.