GB396817A - Improvements in and relating to thermionic electrometers - Google Patents

Abstract

396,817. Pressure meters ; potentiometers ; calibrating-instruments. MORTON, C., Dovedale, Villiers Avenue, Surbiton, Surrey. Feb. 17, 1932, No. 4727. [Class 37.] A null thermionic electrometerfor the measurement of small potential sources having high resistance, for example the pH values of solutions, has the potentiometer current, grid bias, anode voltage, filament current and counter anode continuous current derived from a common source in order to effect complete standardization of the instrument. The main circuit comprises a source of current 1, resistances 2, 3, the valve filament 4, resistances 6, 7, a potentiometer 8 and coarse and fine variable resistances 9, 10. A switch 12 short-circuits resistance 7 or connects a resistance 13 in parallel with the potentiometer 8. The potential to be measured is connected at 14, 15, the pole not connected directly to the grid being tapped on the potentiometer slide-wire ' 18 which has a resistance value equal to two sections of the potentiometer 8, the eight sections of which have equal resistance values. The anode is connected to the source 1 through variable resistances 20, 30, and a double-pole switch 21 is provided to connect the instrument 22 for a test, or for standardization with a standard cell 33. The standardization is effected by setting resistance 3 so that 0.1 ampere flows in the potentiometer 8, the milliammeter being set to zero by adjusting resistances 9, 10. The potentiometer 8 is then adjusted to its zero position with the terminals 14, 15 short-circuited and the instrument reading again brought to zero by adjustment at 29, 31. Thereafter the unknown E.M.F. is read off on the potentiometer 8 which may be calibrated to read pH values directly with compensation for temperature. When pH values are measured, the resistance 13 is paralleled with the potentiometer 8 and the apparatus re-standardized with reference to a solution of known pH value.