994,458. Semi-conductor gating circuits; automatic exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Aug. 24, 1961 [Sept. 7, 1960; April 27, 1961], Nos. 30831/60 and 15290/61. Headings H3T and H4K. In a gating circuit using a semi - conductor breakdown device such as a PNPN diode or triode, a rectifier is connected in series with the signal channel and is provided with current bias circuits at each electrode so as to be normally conducting but, when the signal is of such a magnitude that it would otherwise reduce the current to a value below the holding current of the breakdown device, the rectifier becomes non- conducting whereby the holding current is maintained by the path next to the breakdown device. Fig. 4 shows a coordinate switching system in which a telephone channel sp1 is connected through transformer T1, PNPN diode A, limiter circuit BC, PNPN diode P and transformer T2 to an output circuit SP2. The circuit is set to this condition by applying opposite polarity pulses to terminals ts1 and ts1<SP>1</SP> (switch +Vm being made) and to ts2 and ts2<SP>1</SP> so as to cause breakdown of the PNPN diodes, rectifiers being connected to these terminals to prevent the switching pulses being shunted to earth. Further rectifiers, Rsa, Rsb are connected in series with the signal path and biased through circuits such as L2, L1 to be normally conductive. If the amplitude of the signal current is such that the current through PNPN diodes would be reduced below the holding current, the rectifiers Rfa and Rfb become nonconducting and sufficient current is supplied through inductors L1 and L2<SP>1</SP> to maintain the diodes conducting. The inductors could be replaced by the emitter collector path of a transistor arranged as a constant current device so that the operation would be independent of the resistance of the external circuit (Fig. 6, not shown). Double Zener diodes ZD1 and ZD2 are coupled across the input and output transformers to limit the amplitude of the signal. These may each be replaced by a pair of biased rectifiers or series current limiters (Figs. 3b and c, not shown). Inductors 1c and 1d overcome the delay in operation of the Zener diodes ZD1 and ZD2 and inductors La and Lb compensate for delay in operation of the rectifiers Rfa, Rfb due to hole storage. A switch in the path from the battery plus vm. is operated when the path is ascertained to be the one that is to be selected. A further co-ordinate switching arrangement may be located between the switching devices and the terminal elements in which case the vertical control lines of the circuit shown may be connected to the horizontal control line of the additional arrangement, the voltage pulses appearing at the emitters of the diodes in a further arrangement serving as the required switching on pulses for these diodes. In Figs. 2 and 5 (not shown) PNPN triodes replace the PNPN diodes. Specification 994,460 is referred to.