Basic Features of a DC Potentiometer

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:

• A potentiometer measures the unknown voltage by comparing it with a known voltage source rather than by actual deflection of the pointer. This ensures a high degree of accuracy.
• As a potentiometer measures using null or balance condition, hence no power is needed for the measurement.
• Determination of voltage using potentiometer is quite independent of the source resistance.

A Basic DC Potentiometer

Let’s consider the circuit diagram of a basic dc potentiometer shown below:

Operation of the DC Potentiometer

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.

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