Improvements in or relating to subscribers line circuits for use in telephone systems

811,994. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Oct. 27, 1955 [Nov. 30, 1954], No. 34604/54. Class40 (4). A subscriber's line circuit, having a resistor over which the calling subscriber's loop develops a calling potential, comprises a transistor responsive to this potential. A further transistor is used for repeating ringing current to the line. Completion of the calling subscriber's loop lowers the potential across resistor R1 thereby rendering transistor TX2 conducting. The consequent rise in potential of the point X enables tube VK11. Tens and units scanning pulses are applied to leads TL, UL, as described in Specification 735,635 and when VK1 is enabled, units pulses are passed to tube VK12 which also fires when the appropriate tens pulse is present on lead TL. The resulting pulse is fed to lead LFS to stop the scanning. The striking of VK12 also fires tube VK16 in a preselected link circuit, the rise in cathode potential thereof striking VG4. A voice circuit is now established from the link to the calling subscriber by means not shown and V.F. tone is passed via rectifier MR, which is rendered conducting by the rise in potential of point X, gas diode VG 4 and tube VK16. The link thereupon reverts dial tone. During dial interruptions transistor TX2 becomes non- conducting and the potential of point X falls to cut off rectifier MR, so that the dial impulses are repeated to the link as interruptions of the V.F. tone. In the case where the subscriber is to be rung, tube VK18 is fired, thereby firing the triggering gap of tube VK10. Intermittent current IR of ringing frequency is applied to the anode of VK10 which thus fires on alternate half-cycles, the resulting pulses derived from the cathode resistor being applied to the base of transistor TX1 to cause it to conduct. The resulting collector current pulses are fed to line via transformer TR1 as ringing current. When the called subscriber answers, the rise in potential across R2 cuts of the transistor TX1 to stop the ringing current.