GB878674A - Improvements in or relating to devices for measuring very low voltages - Google Patents

Abstract

878,674. Electric tests. PHILIPS ELECTRICAL INDUSTRIES Ltd. Oct. 9, 1957 [Oct. 12, 1956; March 1, 1957], No. 31580/57. Class 37. In a device for measuring low direct voltages by conversion to an alternating voltage, the direct voltage is applied across a capacitor comprising a movable electrode 3 and a first fixed electrode 1; and a generator 9 applies a highfrequency bias voltage modulated by a lowfrequency voltage between the movable electrode 3 and a second fixed electrode 2, whereby the electrode 3 is caused to vibrate; the resultant alternating version of the low direct voltage developed across the electrodes 1, 2 being fed to a measuring instrument. The capacitor assembly 1, 2, 3 is disposed in a vacuum container; the electrodes are circular, and the movable electrode 3 comprises a synthetic plastic diaphragm bearing a metallic coating and being circumferentially clamped. The generator 9 comprises a high-frequency valve oscillator having the capacitor 2, 3 in the gridcathode circuit; the electrostatic forces between the electrodes 2, 3 causing the electrode 3 to vibrate at its natural frequency thereby modulating the high-frequency voltage generated by the oscillator, Fig. 2 (not shown). The alternating voltage developed across capacitor 1, 2 is proportional, in amplitude, to the input direct voltage from the source 5, and is fed by way of amplifier 24 to the junction of inductors 27, 28; the other end of inductor 28 being fed from generator 9. The resultant voltage has positive and negative peaks, the negative or positive difference of which provides a magnitude and polarity indication of the direct voltage to be measured. This difference voltage is fed to a direct voltage measuring instrument 34 by way of a bridge circuit comprising rectifiers 31, 32 and R.C. circuits 29, 30.