Patent GB-588420-A (GB)
Title: Improvements in electrical apparatus for measuring the potential of an electrostaticcharge
IPC: G01R1/30
Filed: 1945-02-17
Abstract: 588,420. Thermionic valve ammeters and voltmeters; electrification detectors ; resistance measurements. DUNLOP RUBBER CO., Ltd., and BULGIN, D. Feb. 17, 1945, No. 4044. [Class 37] An apparatus for measuring the surface potential, voltage sign and quantity of an electric charge or for measuring the resistance to earth of an object, comprises a tube 6 which is closed at one end and contains an electrometer valve 2, the grid 15 of which is connected to an electrode 8 situated near the open end of the tube, and a meter 12 connected in the anode circuit of the valve. To set the instrument 12, which has its zero marked at a point about 90 per cent of full scale deflection, the pointer is adjusted to zero when the deflection has become stable by means of a variable shunt 13, the grid 15 of the valve 2 attaining a negative potential of 2 volts. Contacts 19 are closed and a negative grid bias supplied from a potentiometer 16 connected across a battery 17 is adjusted to maintain the pointer at zero, so that the potentiometer delivers a voltage equal to the " free " grid voltage which is available for resetting the grid when required ; the contacts 19 are then opened. The open end of the tube 6 upon which the meter 12 is mounted, is pointed at the voltage source to be measured and. at a distance from the source such that an angle of not less than 90 degrees is subtended therefrom. If the meter reading decreases immediately to a stable value and returns to zero on removal of the apparatus from the field of the source, then the source is negative. If the charge is positive, the pointer of the meter 12 moves off the scale; to measure the voltage of this charge, the grid is earthed by closing and then opening the switch contacts 19 and then removing the apparatus from the field, thus producing a negative charge on the grid 15 and consequently a meter reading. If the voltage is alternating, the meter reading decreases more slowly than with a negative charge, due to the flow of grid current during the positive portion of the cycle, and the grid retains its negative charge when the apparatus is removed from the field. The value of the voltage is obtained by multiplying the meter reading by the distance in feet from the source and by a multiplier factor if the open end of the tube is partly closed by a metal cap having a central aperture. The quantity of charge in coulombs per square centimetre of surface.is obtained by measurement of the voltage across a condenser of known capacitance, which is pressed against the surface under test and is spaced from the end of the tube 6 by three onecentimetre distance pieces, by means of a probe, of which one end is connected to the grid 15 and the other is coterminous with the open end of the tube. An object, the resistance to earth of which is to be measured, is connected in parallel with a condenser of known capacitance in the grid circuit. The contacts 19 are closed and the potentiometer 16 is adjusted until the meter indicates 4.1 volts. The contacts 19 are then opened and the time taken for the voltage to drop to 2.5 is noted ; from this time the value of the resistance can be obtained. According to the Provisional Specification, the apparatus may be provided with optical means such as a range-finder for determining the distance between the electrode and the object.