GB199483A - Improvements in the measurement of alternating potential differences - Google Patents

Abstract

199,483. Moullin, E. B. March 25, 1922. Pressure meters.-A direct-reading voltmeter for alternating electric potentials, and particularly for potentials below 3 volts, comprises a 3-electrode thermionic valve with asymmetrical conductivity between the anode and filament, the potential being applied between terminals AB connected to the grid and filament, and the increase in mean anode current, due to the applied potential and to the curvature of the characteristic of the valve, being measured by a galvanometer G connected between the positive side of the filament and the anode and calibrated to read the applied potential directly. The terminal B may be connected to a fixed point of the variable filament resistance S at a potential of 1.6 volts negative with respect to the negative side of the filament. The anode circuit includes no battery, the potentials of the anode, grid, and filament being all controlled by the resistance S. In calibrating the instrument, the zero of the galvanometer scale is taken at the reading obtained when the terminals A, B are connected together and when the resistance S is adjusted until the potential drop across the filament is about 3.6 volts. Standard conditions are obtained in use by adjusting the resistance S until a zero reading is obtained with the terminals A, B connected together. A high resistance R of the order of 30,000 ohms is included in the anode circuit in order to obtain galvanometer deflections more nearly proportional to the applied potentials, the resistance R decreasing the mean anode potential when the applied potential, and consequently the anode current, increases, and thu spreventing too rapid increases in the mean anode current. A condenser C of the order of 1 microfarad connected in shunt to the galvanometer G and resistance R provides a path of small impedance for the alternating component of the anode current and prevents fluctuations of the anode potential, thus giving readings independent of frequency changes. In a modification, the grid is maintained at the required negative potential by a separate cell between the terminal B and the negative side of the filament.