Improvements relating to automatic telephone systems

890,109. Automatic exchange systems; circuits employing bi-stable magnetic elements. ERICSSON TELEPHONES Ltd. Oct. 12, 1960 [Oct. 16, 1959], No. 35182/59. Classes 40 (4) and 40 (9). A magnetic core N having a rectangular hysteresis characteristic and provided with input, read and output windings W1, W3, W4 is used to detect the free or busy condition of a subscriber's line in an automatic exchange system, the core magnetization remaining in a datum remanent state when the line is free so that read pulses are unable to produce output pulses, and the core magnetization being reversed by line current when the line is brought into use whereby each read pulse momentarily returns the magnetization to the datum state against the bias action of the line current, output pulses being then produced for the duration of any interval when the line is busy. In Fig. 1, each subscriber I, II has a separate line circuit associated by two input windings W1, W2 with a magnetic core N. Coupling to the exchange is effected over a transformer having windings T1, T2, T3 and over a symmetrical transistor S. Each core is read repeatedly by pulses applied from terminals P to the respective read windings W3, the pulses applied to respective cores being in different time positions. If a line is calling or busy, output pulses are induced in a circuit R common to the output windings W4, and the time position of these pulses is indicative of the line circuit in use. Dialling pulses are indicated in the output circuit as interruptions of the train of pulses. In an application to party line calling, sharing subscribers IX, IY and IIX, IIY have individual cores NX, NY associated respectively with the negative and positive line wires. The read winding circuit P is common to the two cores of a shared line circuit and each core has a separate output circuit RX, RY. When the line circuit is busy, output pulses appear in both circuits RX and RY, the time position of the two pulse trains being indicative of the line circuit in use. When the line circuit is in the calling condition pulses appear only in one or other of the output circuits, depending on which of the sharing subscribers is calling, while the shared service call button CBX is depressed at the calling subscriber's substation. If the line is calling, and while button CBX is depressed, the pulse train in circuit RX or RY opens a 1-gate G1 after shaping in SS and indicates at OB and OC that a register is required. When CBX is released bo