Patent GB-644742-A (GB)
Title: Improvements in or relating to electrical measuring apparatus
IPC: G01R15/00
Filed: 1948-08-18
Abstract: 644,742. Thermionic valve voltmeters. GENERAL ELECTRIC CO., Ltd., and COLLINS, L. Aug. 18, 1948, No. 21816. [Class 37] A thermionic valve voltmeter is provided with means adapted to prevent those values of voltage applied to the control grid of the valve which lie outside a predetermined range from causing a deflection greater or less than that caused by voltages within this range. A triode 1 in series with a resistance 2 forms one arm of a Wheatstone bridge, the other arms of which comprise resistances 3, 4, 5, 6, 7. A hightension D.C. supply 8 is connected across one diagonal of the bridge so that the cathode 9 is connected to the negative terminal and the anode 10 is connected to the positive terminal through the resistances 2, 3. A voltmeter 12 is connected across the other diagonal of the bridge, which includes a wiper arm 11 by which the bridge may be balanced. The anode 14 of one diode 15 of a double-diode valve 13 is connected through a load resistance 16 to the negative terminal of the supply 8. The cathode 17 of the diode 15 is connected to a point on the potentiometer formed by resistances 18, 19, 20 which is positive with respect to the negative terminal of the supply, thus making the cathode 17 initially positive to the anode 14. This anode 14 is connected through a low-pass filter 21, 22 to the grid 23 of the triode 1. The anode 24 of the second diode 25 is connected to the anode 10 of the triode 1, whilst the cathode 26 is connected to a tapping point 27 on the resistance 6 so as to be at a potential positive to that of the negative supply terminal. An alternating voltage to be measured is applied across the load resistance 16 and if the peak voltage is greater than the negative bias of the anode 14 with respect to the cathode 17, a unidirectional voltage is developed across the resistance 16 due to the diode current and makes the grid 23 more negative with respect to the cathode 9. If the bridge circuit was previously balanced this results in a deflection of the meter 12. As the grid 23 becomes more negative, the anode 10 becomes more positive with respect to the cathode 9 and it is arranged that when the voltage to be measured exceeds that which is sufficient to cause the maximum desired deflection of the meter 12, the potential of the anode 10 is greater than that of the cathode 26 of the second diode 25. This becomes conductive and prevents the anode 10 from becoming more positive than the cathode 26 of the second diode 25, so that there is no increased deflection of the meter 12. The voltmeter 12 may be provided with a scale corresponding to a desired fraction of the range of voltage that may be applied to the grid 23 of the valve 1 without voltages outside this fraction of the range causing damage to the voltmeter. The accuracy of measurements within the desired fraction can then be substantially increased. The apparatus may be used in the power-level measuring apparatus described in Specification 644,741, the voltmeter 12 then being a centre-zero instrument with a minimum deflection corresponding to minus one decibel and a maximum deflection corresponding to plus one decibel.