Improvements in or relating to variable electrical equalisers

903,691. Variable equalizers. GENERAL ELECTRIC CO. Ltd. Nov. 9, 1960 [Nov. 11, 1959], No. 38241/59. Class 40 (4). In a variable equalizer of the type comprising two impedances the product of which is required to be kept constant, these impedances are connected in the arms of an almost balanced bridge or like circuit one of the impedances being controlled by the unbalance current of the bridge. As shown, the equalizer consists of a T-network 7 whose shunt arm is connected to a network 12 terminated by a thermistor 13, and whose series arms are shunted by a further network 11 terminated by a further thermistor 14, of the type described in Specification 508,341. The thermistors 13, 14 which have negative temperature coefficients are contained in opposite arms of a Wheatstone bridge 20 whose diagonals form the input and output of an amplifier 34, 35, the bridge being arranged to be slightly out of balance to produce sufficient feedback to cause the amplifier to oscillate at a frequency (5 kc/s.) low compared with the signal currents. When a control current applied to terminals 5, 6 changes in such a way as to increase the resistance 13 the bridge becomes more out of balance to increase the amplitude of the oscillation, which oscillation after amplification by transistor 48 controls the heating of the thermistor 14 to decrease its resistance. Filters 52, 53, 54, 55 prevent the oscillator current from entering the signal path and the signal current from entering the bridge circuit. In a modification the bridge is energized by D.C., the out-of-balance D.C. current being used to control amplification means for providing heater current to the thermistor 14. Such means may comprise an oscillator having a modulator in the feedback circuit controlled by the out-of-balance current as described in Specification 873,786.