# Questions on Potentiometer

## What is the principle of a potentiometer?

The potentiometer is a device used as an adjustable voltage divider. It has a resistance of uniform cross-section. A voltage is applied between two endpoints of the resistance. As the resistance has a uniform cross-section, the voltage drop per unit length of the resistance is equal. Now we connect one terminal to one end of the resistance and one sliding contact on the resistance which can be wiped. Now if we measure the voltage between the resistance end terminal and sliding terminal we get a voltage which is directly proportional to the distance of the sliding contact tip on the resistance from resistance end terminal. So by sliding the moving contact on the resistance we can get desired voltage between the output terminals. This is the most basic working principle of the potentiometer.

## What are the types of potentiometer?

Depending on the mechanism of operation the potentiometer can be categorised in two basic types. These are the mechanical type and digital type. In mechanical type potentiometer, the sliding contact on the resistance elements can be moved mechanically. The output voltage of the potentiometer depends on the position of the sliding contact on the resistance. The working of a digital potentiometer is somewhat different from the conventional mechanical potentiometer. In digital potentiometer, a number of resistive elements are connected in series. Semiconductor switches are connected between each resistive element. The other end of all the switches is connected to a logic circuit through which the switches are controlled. Only two of the switches can be made on at a time. Depending on the number of resistive elements between two on switches, the device gives the output voltage. The input voltage is applied across the entire series of resistive elements.

The mechanically operated potentiometer can also be a rotary type and linearly sliding type. In rotary type potentiometer, the wiping contact wipes on a semi-circular resistive element to get desired voltage between one fixed and the wiping contact. In linear sliding potentiometer, the sliding contact gets moved on the resistor for adjusting voltage output between an end terminal of the potentiometer resistor and the sliding contact.

## What does a potentiometer measure?

The potentiometer mainly measures the voltage between two points. The main advantage of measuring voltage by a potentiometer is that it does not take any current from the circuit across which the voltage is to be measured. So there is no change in the real voltage during measurement. So potentiometer gives much more accurate measurement than a voltmeter. Measurement with a potentiometer is more laborious than with a voltmeter but it is still used when the accuracy of measurement is the main requirement. The potentiometer is used to measure the internal resistance of a battery cell. It is also used to compare the emf of a battery cell with a standard cell. But all those measurements are basically on the principle of Voltage measurement by a potentiometer.

## What is the difference between a rheostat and 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.

## How do you calculate the sensitivity of a potentiometer?

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.

## What is driver cell in potentiometer?

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.

## Why is potentiometer is preferred over voltmeter?

Both potentiometer and voltmeter are used for measuring voltage. But potentiometer gives more accurate measurement than a voltmeter. This is because, potentiometer does not take any current from the circuit hence there is no change in the voltage drop across the branch of a circuit across which the potentiometer measures the voltage. But when a voltmeter is connected across the branch for measuring voltage, it takes a non zero current from the circuit which obviously affects the voltage to be measured across the branch.