Measurement of Voltage by Potentiometer

The principle of measuring voltage across a branch of a circuit with help of a potentiometer is also simple. Here first we have to adjust the rheostat to adjust the current through the resistor so that it causes a specific voltage drop per unit length of the resistor. Now we have to connect one end of the branch to the beginning of the resistor and other end is connected to the sliding contact of the resistor through a galvanometer. Now we have to slide the sliding contact on the resistor until the galvanometer shows zero deflection. When the galvanometer comes to its null condition we have to take the reading of the position of the sliding contact tip on the resistor scale and accordingly we can find out the voltage across the branch of the circuit since we have already adjusted the voltage per unit length of the resistor.

Rheostat vs Potentiometer

A potentiometer gives variable voltage. A rheostat gives variable resistance. The potentiometer is a three terminal device whereas a rheostat is a two terminal device. Construction wise both of the devices look similar but their principle of operation is entirely different. In potentiometer two end terminals of the uniform resistance are connected to the source circuit. In rheostat, only one terminal of the uniform resistance is connected to the circuit and the other end of the resistance is kept open. In both potentiometer and rheostat, there is a sliding contact on the resistance.

In potentiometer, the output voltage is taken between fixed and sliding contact. In rheostat, the variable resistance is achieved between fixed and sliding terminal. The resistance of potentiometer gets connected across the circuit. The resistance of rheostat is connected in series with the circuit. The rheostat is generally used to control the current by adjusting resistance with the help of sliding contact. In potentiometer, the voltage is controlled by adjusting the sliding contact on the resistance.

tat, the variable resistance is achieved between fixed and sliding terminal. The resistance of potentiometer gets connected across the circuit. The resistance of rheostat is connected in series with the circuit. The rheostat is generally used to control the current by adjusting resistance with the help of sliding contact. In potentiometer, the voltage is controlled by adjusting the sliding contact on the resistance.

Potentiometer Driver Cell

The potentiometer measures voltage by comparing the measuring voltage with voltage across the resistance of the potentiometer. So for operation of potentiometer there must be a source voltage connected across the potentiometer circuit. This cell to provide this source voltage to drive the potentiometer is called driver cell. The driver cell delivers the current through the resistance of potentiometer. The product of this current and the resistance of the potentiometer provides full scale voltage of the device. By adjusting this voltage one can change the sensitivity of the potentiometer. This is normally done by adjusting current through the resistance. The current flowing through the resistance is controlled by a rheostat connected in series with the driver cell. This is to be remembered that the voltage of the driver cell must be greater than the voltage to be measured.

Potentiometer Sensitivity

The sensitivity of a potentiometer implies what the small voltage difference can be measured by the potentiometer. For same driver voltage if we increase the length of the potentiometer resistance, length of the resistance per unit voltage gets increased. Hence the sensitivity of the potentiometer gets increased. So we can say sensitivity of a potentiometer is directly proportional to the length of the resistance. Again if we reduce the driver voltage for a fixed length of potentiometer resistance, then also voltage per unit length of the resistance gets decreased. Hence again the sensitivity of the potentiometer gets increased. So the sensitivity of the potentiometer is inversely proportional to the driver voltage.