Application of the Electrometer Triode to the Measurement of High Resistance

Abstract

IN some experiments made by us on the application of the electrometer triode to the measurement of the ionisation produced by radioactive substance and X-rays, alcohol xylol resistances of the order 10³–10⁶ megohms have been employed. A method of measuring such resistances has been devised, in which use is made of the very high grid-filament resistance of the triode. The arrangement is indicated in Fig. 1. R₁ is a known resistance, R₂ is the resistance to be determined. With the potentiometer short-circuited and the key K open, the normal anode current is first balanced by variation of the resistance Z in the short-circuit GZ, so that the galvanometer indicates zero. The key K is now closed, and the current in the circuit ER₁ R₂ alters the voltage of the grid by an amount equal to the voltage drop across R₁. The anode current is thereby altered, and the galvanometer is deflected. The grid is now restored to its original potential, by applying to it a potential by means of the potentiometer P, the reading of the potentiometer e being a measure of the voltage across R₁ Obviously then Resistances up to 10⁶ megohms have been measured in this manner, the procedure being to construct a series of alcohol-xylol resistances of the order 10², 10³, 10⁴, and 10⁵ megohms. The lowest of these is first compared with a standard resistance of 10 megohms. This is then used as a standard to measure the 10³ megohms, the process being repeated for the higher resistance.