844,053. Automatic telephone exchanges; circuits of bi-stable magnetic elements. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. April 29, 1957 [April 28, 1956], No 13095/56. Classes 40 (4) and 40 (9). In an arrangement for repeating D.C. impulses, the D.C. is applied to one winding of an inductor, having a core of material which has a substantially rectangular hysteresis loop, to change the magnetization of the core material from a first condition substantially of saturation in one direction to a second condition substantially of saturation in the opposite direction, means being provided which, on the cessation of the D.C., cause the core material to revert to the first condition, and the changes in flux in the core between the first and second conditions control the operation of a bi-stable toggle circuit which produces a D.C. output in one of its states. Such an arrangement results in the binary circuit being isolated from the source of incoming D.C. impulses as regards high-voltage surges, and may be employed to repeat loop impulses from a telephone subscriber's line to exchange apparatus, such as transistors, which might be damaged by such surges even although the usual protective devices were provided, due to the latter not being sufficiently rapid in operation. General description, Figs. 1, 2.-The inductor M has four windings, the input coil T1, two output coils T2, T3 connected to the toggle circuit TOG, and a reset coil T4 connected to the toggle circuit through delay means DEL. The toggle circuit is normally in the " O " condition and the core is in the reset condition with remanent flux density - BR (Fig. 2). Current flowing in T1 changes the flux density to + BMAX, thus producing a negative current pulse in T2 and a positive one in T3. The latter pulse is suppressed but the former transposes the toggle circuit to the " 1 " condition and an output may be taken from the " 1 " side of the toggle. Change-over of the toggle circuit also causes current flow in T4 which tends to reduce the flux density from the value + BMAX. While the current in T1 persists the flux density is not reduced below + BR; but when it ceases, the current flow in T4 causes the flux density to change to - BMAX. The resulting positive pulse in T2 is suppressed but the negative pulse in T3 restores the toggle circuit to condition " O," thus preventing further current flow in T4, whereupon the flux density reverts to its original value of - BR. The delay means DEL prevents rapid growth of the current in T4 which might otherwise result in sufficient current being induced in T1 to cause spurious resetting of the toggle circuit. Detailed description. Fig. 3.-With the subscriber's line unlooped the core is in the reset condition (- BR) and the toggle circuit is in the " O " condition with transistor TX1 conducting and TX2 cut off. Current flow in coil T1, which occurs when the line is looped, changes the flux density to + BMAX. The resulting positive current pulse in T3 is ineffective (MR2 non- conductive), but the negative current pulse in T2 causes current to flow from ground through R10. The junction of R8 and R10 goes negative, whereupon TX2 conducts and raises the junction of R4 and R6 to ground potential, thus cutting off TX1. When the current flow in TX1 ceases, the potential at the junction of R3 and R13 changes from ground to that of negative battery, whereupon TX3 conducts and its collector current flows through T4 in series with the delaying choke L1. The flux density changes to + BR and remains such until a disconnection occurs in the loop due for instance to the subscriber dialling or hanging up. Current then ceases to flow in T1 and the current flow in T4 is effective to change the flux density to - BMAX. The positive pulse produced in T2 is ineffective but the negative pulse in T3 causes the potential at the junction of R7 and R9 to go negative, whereupon TX1 conducts and cuts off TX2. Transistor TX3 is also cut off, current flow in T4 ceases, and the flux density reverts to - BR. Looping and unlooping of the line thus produces a corresponding waveform at the output lead OUT. If the invention is used in conjunction with existing subscribers' instruments it is necessary to connect a resistor of about 1000 ohms across the transmitter of the handset in order to prevent breaks in the loop circuit which might occur due to agitation of the carbon granules of the microphone on positioning of the handset, from producing unwanted changes in the inductor flux density. The core is toroidal with the four coils mounted at opposite ends of two perpendicular diameters, the two output coils being at opposite ends of one diameter, and satisfactory results have been obtained with a toroid of inner diameter 0.5 inch and outer diameter 0.75 inch.