734,028. Measuring bridges for voltage levels. STANDARD TELEPHONES & CABLES, Ltd. Oct. 24, 1952 [Nov. 8, 1951], No. 26737/52. Class 37. [Also in Group XL (c)] In a device for detecting variations of an electrical quantity such as voltage to values beyond predetermined margins wherein such quantity is applied to detecting means delivering a substantially constant output current to relay means while the quantity remains within marginal values, and the output current changes sharply to a second or third distinctive value when either marginal value is reached to immediately operate the relay means, there is provided an oscillator having circuit elements determining alternative operating frequencies f1, f2 and a single oscillating element; the oscillator being quiescent for an input quantity within the marginal values at a steady output current, and oscillatory at one or other frequency as the quantity reaches the respective marginal values to pass sharply distinguished values of output currents, whereby relay means are operative in one or other of two ways to increase or decrease the input quantity and/or to operate an alarm signal. In Fig. 1 the anode circuit of a pentode oscillator VA contains seriesconnected primaries of transformers T1 and T2 tuned to frequencies f1 and f2 by condensers C1 and C2 and the secondary windings energize Wheatstone bridges B1 and B2 respectively comprising resistors R3, R4, R5, and R6, R7, R8, condensers C5 and C6, and thermally variable non-linear resistances (such as barretters) R1 and R2. The bridge outputs are serially coupled in the grid-cathode circuit of the pentode by transformers T3, T4 whose secondaries are tuned to frequencies f1, f2 respectively by condensers C3, C4 and an input alternating voltage whose variations are to be monitored is coupled through transformer T5 to directly energize the non-linear resistances. When the input voltage lies within marginal values predetermined by the adjustments of the variable arms R5, R8 of bridges B1, B2 the circuit is non-oscillatory due to the attenuation of each bridge exceeding the gain of the pentode, whose anode current has a value Io determined by the cathode bias resistor R10, and cathode relay Ar1 is operated to open its normallyclosed contacts while relay Ar2 is inoperative so that its contacts remain open. When the input rises above the present marginal level, non- linear resistance R2 reaches a value permitting positive feed-back over bridge B2 while the value of R1 is such as to prevent feed-back over bridge B1. Oscillations start at frequency f2, and a further secondary of transformer T2 energizes a rectifier RE2 to develop a positive bias voltage across load circuit C8, R9 to increase the anode current of the oscillator pentode to I2>I0, whereby relay Ar2 is operated to close its contacts and operate an alarm or control circuit. Similarly when the input voltage falls below the preset marginal level the value of R2 is such as to prevent positive feed-back over bridge B2 while that of R1 becomes such as to permit positive feed-back over bridge B1 so that the pentode oscillates at frequency f1. Rectifier RE1 energized by a further secondary of transformer T1 develops a negative bias across load circuit C7, R13 to reduce the oscillator anode current to I1<I0, whereby relay Ar1 is inoperative to close its contacts in the alarm or control circuit. The negative bias is limited to a value which will still permit oscillations by the antiresonant circuit L1, C10 tuned to frequency f1 inserted in the oscillator cathode circuit across C7, R13. Normally this will produce degenerative cathode feed-back reducing the amplitude of oscillation at frequeney f1 but as the negative bias across C7, R13 increases, the rectifier RE3 shunted across L1, C10 will be rendered increasingly conductive to progressively reduce the degeneration and maintain the pentode in oscillation.