Electronic Bridge for Use with the Glass Electrode *

This apparatus extends the sensitivity of an ordinary potentiometer and galvanometer so that accurate measurements can be made of the electromotive force of a cell containing a glass electrode having a resistance of about 100 megohms. The electronic bridge serves the same purpose as other circuits which have been described ;2 its advantages are good stability, high sensitivity, and moderate cost. The bridge circuit, shown in Figure 1, is based on the use of two 959 "acorn" pentodes, as in a circuit described by Burr and Mauro.' These tubes are employed as "space charge grid" pentodes to reduce the grid current to less than 1013 amperes;' one tube is used actively, the other passively. The resistance arms of the bridge consist of two fixed resistors of precision type, 2,500 ohms each, supplemented by a variable component consisting of a 20,000 ohm Helipot unit shunted by a 5,000 ohm resistor. In order to ensure maximum stability, only wire-wound resistors are used. The zero setting of the galvanometer is adjusted by the Helipot. Experience with various insulating materials demonstrated the superiority of Lucite, which was therefore employed for the socket bases and the input switch (SW in Fig. 1). Each Lucite socket was constructed by using clips removed from a standard ceramic socket. The input switch is described in detail in Figure 2. The spring leaves were adjusted so that contact B is made before contact A is broken in order to minimize the sudden deflection or kick of the galvanometer which occurs when the circuit is opened even for an instant. The switch is operated by deflecting the center arm Y in a horizontal plane by means of a Lucite rod extending through the front panel of the case. The switch is wired so that contact A is grounded, contact B is connected to the glass electrode, and the center arm is connected to grid No. 3 of the active tube. Since the potential differences from the glass electrode cell and the potentiometer are opposed, only the difference