Potentiometer basic circuit

Fig. 5.15 Basic circuit for the electrode potential measurement during current flow A is working (indicator, test), B, auxiliary and R, reference electrode connected by means of the Luggin capillary (arrow) and P, potentiometer. 

The pH meter is basically an electronic voltmeter (or potentiometer) designed for use with a glass electrode system. It is composed of (i) a reference electrode, (ii) a glass electrode responsive to the pH of the solution surrounding it, and (Hi) an electrometer, which is a device capable of measuring very small differences in electrical potentials in a circuit of extremely high resistance. 

The potentiometer, used for accurate determinations of voltage in standards laboratories, has various applications in biological research. One, of course, is the accurate determination of various bioelectric potentials. In addition, many instruments can be constructed about the potentiometer as a central core. Figure 5.10 illustrates the basic potentiometer circuit. The instrument consists of two batteries, a standard reference cell and a working battery. In some instrumentation systems, the working battery is replaced by a regulated power supply, and a zener diode reference source is used in place of the standard cell. The heart of the instrument is a very accurately calibrated resistance, which is called a slide wire. It is usually in the form of a helix wrapped about a solid core. In operation, switch 1 is connected to the standard cell. The key (switch 2) is tapped at intervals... 

The basic electrical circuit is a simple DC Wheatstone bridge incorporating in one arm a thermistor of nominal resistance 10 kohm at 25 C. The off-balance voltage is recorded on a recording potentiometer (10 mV full-scale deflection) with a chart speed of 4 cm/min. 

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