A potentiometer is an instrument that is used for measurement of potential difference across a known resistance or between two terminals of a circuit or network of known characteristics. A potentiometer is also used for comparing the emf of two cells. A potentiometer is widely used in measurements where the precision required is higher than that obtained by ordinary deflecting instruments or where no current is required to be drawn from the source under test, or where the current must be limited to a small value.
Because a potentiometer measures voltage by comparing it with a standard cell, it can also be used to measure the current simply by measuring the voltage drop produced by the unknown current passing through a known standard resistance.
Using a potentiometer, power can also be calculated and if time is also measured, energy can be determined by multiplying the power and time of measurement. Therefore, a potentiometer is one of the essential instruments of electrical measurements.
Key features of potentiometers are:
Let’s consider the circuit diagram of a basic dc potentiometer shown below:
First, the switch S is put in the ‘operate’ position and the galvanometer key K is kept open, the battery supplies the working current through the rheostat and the slide wire. The working current through the slide wire may be varied by changing the rheostat setting. The method of measuring the unknown voltage E1, depends upon finding a position for the sliding contact such the galvanometer shows zero deflection, that is, it indicates null condition, when the galvanometer key K is closed. Zero galvanometer deflection implies that the unknown voltage E1 is equal to the voltage drop E2, across position a-c of the slide wire. Therefore, the determination of the values of unknown voltage now becomes a matter of evaluating the voltage drop E2 along the portion a-c of the slide wire.
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When the switch S is placed at ‘calibrate’ position a standard or reference cell is connected to the circuit. This reference cell is used to standardize the potentiometer. The slide wire has a uniform cross-section and hence uniform resistance along its entire length. A calibrated scale in cm and fractions of cm is placed along the slide wire so that the potentiometer sliding contact can be placed accurately at any desired position along the slide wire. Since the resistance of the slide wire is known accurately, the voltage drop along the slide wire can be controlled by adjusting the values of working current. The process of adjusting the working current so as to match the voltage drop across a portion of sliding wire against a standard reference is termed as ‘standardization’.
You can also read: Power Measurements in DC Circuits
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