Condenser Technique for Measuring Glass Cell or Other Potentials in Circuits of High Resistance

Abstract

Glass cell potentials or other electrode potentials in series with high resistance can be measured conveniently by a modification of Beans and Oaks' 1 condenser technique. Instead of the simple ballistic method employed by them, the cell is compensated by a potentiometer, and a null point reading made, thereby reducing polarization of the cell by reducing the charge on the condenser (Fig. 1).

The Leeds & Northrup type K potentiometer lends itself readily to such an arrangement. Glass cell and condenser are connected in series to the regular E.M.F. binding posts (Fig. 2). From a point between glass cell and condenser a lead passes to the insulated lower contact of the key K and from the upper contact to the galvanometer binding post proximal to the E.M.F. posts. The double point, double throw switch of the potentiometer is short-circuited across its front posts, at S, and the potentiometer may still be checked against the standard cell in the usual manner without complication. The potentiometer tap keys with resistance protection (K₁) are employed as usual in reading the standard cell check, and the extra tap key K only is used in reading the glass cell-potentiometer balance, the condenser acting as a protection to the galvanometer.

A 2 m.f. condenser is sufficient to give a reading to 0.2 millivolt on a Leeds & Northrup high sensitivity galvanometer (10^-10 amp./mm.) critically damped by a shunt. With a glass cell of 150 megohms resistance, the time of charge to 1/2 final voltage of the condenser through this resistance is 3.5 minutes, the time to 90% is 12 minutes. Adjustment to within one millivolt of the null point can be made by readings at intervals of only a few seconds, since the condenser need be charged to only a small fraction of its full capacity to obtain this sensitivity.