What is the Use of Potentiometer?
A potentiometer is a three-terminal variable resistor. A resistor, a bundle of resistance, is one of the commonly used components in an electric circuit. Often used to regulate the current flow either by adding or subtracting resistance from the circuit, resistors are available in several different shapes and sizes. Apart from the classification based on these two features, i.e., size and shape, the resistors are categorized as fixed and variable. As the names suggest, a fixed resistor has a fixed value of resistance, whereas a variable resistor possesses a resistance value over a defined range.
Out of the various variable resistors available, the potentiometer is the most common one. In this article, we will discuss working, applications, and many other things related to the potentiometer. Nevertheless, to explain or understand the potentiometer working, we first need to comprehend the potentiometer. So, let's start.
What is a Potentiometer?
As discussed above, a Potentiometer is a resistor with three terminals. Out of the three terminals – one is varying, whereas the rest two are fixed. It is an electric component used to measure the unknown voltage by comparing it with the known one, which can be drawn by a cell or any of the other supply sources. In other words, it is a device used for measuring the EMF (electromotive force), and internal resistance of a given cell. A Potentiometer can also be used to vary the resistance for measuring the potential differences. Although the most common use of Potentiometer is to control volume in radios and other audio-related electronic equipment, they are also helpful in manufacturing electronic components that provide a way of adjusting circuits to obtain the correct outputs.
Characteristics of Potentiometer
Some significant characteristics of Potentiometer are as follows:
As the potentiometers work on the comparative method rather than the deflection method for determining the unknown voltages, these are very accurate.
Potentiometers measure null or balance point that doesn't require the power for measurement.
As no current flows through pots when they are balanced, their working is free from the source resistance.
Potentiometer Working Principle
The basic potentiometer working principle is based on the fact that the potential across any piece of the wire is directly proportional to the length of the wire, which has a uniform cross-sectional area and the constant current flowing through it. In the case of no potential difference amid any two notes, the electric current will flow. The working principle of pot can be understood and explained through the below circuit, where L is the long resistive wire. The circuit also includes a battery of known EMF (electromotive force) and V, voltage named as driver cell voltage. Now, by connecting battery terminals to the two ends of L, assume a primary circuit arrangement. One end of this circuit is connected to a galvanometer (G) and the other to the cell whose electromotive force (E) is to be measured. The new circuit formed now is considered to be a secondary circuit.
Below is the derivation to explain the working principle of Potentiometer:
According to Ohm's law,
V = IR
Where I is current, R is total resistance, and V is voltage.
Now, R = ρL/A and V = I (pL/A).
Where ρ is resistivity and A is the cross-sectional area.
With ⍴ and A constant, I is also constant for a rheostat. Hence,
Lρ/A = K
V = KL
E = Lρx/A =Kx
Where x: length of Potentiometer wire, E: cell with Lower EMF, and K: constant.
As the potential difference is zero (0) and there's no flow of current, the galvanometer has null detection. So, x is considered as the length of the null point. Ultimately, the unknown electromotive force can be found by using K and x, as shown below:
E = Lρx/A = Kx
Since the EMF here includes two cells, let L1 and L2 be the null point length of the first cell and second cell with EMF E1 E1 and EMF E2 accordingly. So,
\[ \frac{E_{1}}{E_{2}} = \frac{L_{1}}{L_{2}} \]
Types of Potentiometer
Although the basic working principle and construction of all potentiometers are the same, they differ based on the geometry of moving terminals. Most of the potentiometers have a wiper that rotates on an arc-shaped resistive material. Nonetheless, in some other types of potentiometers, the wiper slides linearly on a straight resistive strip. Based on the concept of the resistive strip, the potentiometers are of two types: Rotary and Linear.
Rotary Potentiometer: This sort of pot has a wiper that rotates across two terminals for varying the resistance of the potentiometer. Depending upon the number of times one can turn or rotate the wiper, rotary potentiometers can be classified in the below categories:
Single Turn: The wiper takes only a single turn and often rotates just 3/4th of the complete turn.
Multi-Turn: These potentiometers can make multiple rotations such as 5, 10, or more.
Concentric Pot: Here, two pots are adjusted together by using concentric shafts.
Servo Pot: This is a motorized pot used to adjust or control a servo motor automatically.
Linear Potentiometer: Also known as slider, fader, or slide pot, these are potentiometers in which the wiper slides on a straight resistive strip. These can further be classified in the following categories:
Slide Pot: Slide potentiometers are the high-quality pots mostly made from conductive plastics.
Dual Slide Pot: This sort of pot is the calibration of two slide pots in parallel.
Multi-Turn Pot: This type of potentiometer is constructed by using a spindle, which actuates the slider.
Motorised Fader: The resistance and movement of the wiper in this pot are controlled by a motor.
Applications of Potentiometer
A potentiometer operates as a voltage divider and, therefore, has numerous applications. Some of the applications of pots are as follows:
Audio Control: Both rotary and linear Potentiometers are used to control audio devices for changing and controlling the loudness and other audio-related signals.
Television: In televisions, the pots are used to control the brightness, colour, and contrast of the picture.
Motion Control: Pots are also used as servomechanisms, the position feedback devices used to create a closed-loop control.
Transducers: Due to the aspect of giving large output signals, pots find applications in designing displacement transducers.
FAQs on Potentiometer Working - Principle & Diagram
1. What is the working principle of a potentiometer?
The working principle of a potentiometer is that the potential drop (V) across any length (l) of a wire with a uniform cross-sectional area and through which a constant current (I) flows, is directly proportional to that length. This fundamental relationship is expressed as V ∝ l. This allows for the precise comparison of potential differences by measuring balancing lengths.
2. How is a standard potentiometer constructed for a physics lab?
A standard potentiometer consists of a long resistive wire, typically made of an alloy like Manganin or Constantan due to their low temperature coefficient of resistance, stretched over a wooden board. The wire is usually several metres long and arranged in parallel segments to fit in a compact space. It is connected in series with a strong power source called a driver cell, a key, and a rheostat to maintain a constant current in the primary circuit.
3. What are the main components shown in a potentiometer's circuit diagram?
A typical potentiometer circuit diagram for an experiment includes the following main components:
- Primary Circuit: This contains the driver cell (or battery), a plug key, and a rheostat. These are connected in series to the two ends of the potentiometer wire.
- Secondary Circuit: This includes the cell whose EMF is to be measured, a high-sensitivity galvanometer to detect the null point, and a jockey (a sliding contact) to move along the wire.
- Potentiometer Wire (AB): This is the long, uniform resistance wire where the potential drop occurs.
4. Why is a potentiometer preferred over a voltmeter for accurately measuring the EMF of a cell?
A potentiometer is preferred over a voltmeter because it measures the EMF of a cell under a 'null deflection' condition. At the balance point, the potentiometer draws no current from the cell being measured, thus measuring its true EMF. In contrast, a voltmeter always draws some current to operate, which causes a potential drop across the cell's internal resistance. Therefore, a voltmeter measures the terminal potential difference (V = E - Ir), which is always slightly less than the actual EMF (E).
5. What are the two main applications of a potentiometer according to the CBSE 2025-26 syllabus?
As per the CBSE 2025-26 syllabus for Class 12 Physics, the two primary applications of a potentiometer are:
- Comparing the EMFs of two primary cells: By finding the balancing lengths (l₁ and l₂) for each cell, their EMFs (E₁ and E₂) can be compared using the direct relation E₁/E₂ = l₁/l₂.
- Determining the internal resistance of a given primary cell: By using a resistance box and finding balancing lengths with and without a shunt resistor, the cell's internal resistance (r) can be accurately calculated.
6. How does the length of a potentiometer wire affect its sensitivity?
The sensitivity of a potentiometer is inversely proportional to its potential gradient (K), which is the potential drop per unit length. A more sensitive instrument can detect smaller changes in potential difference. By using a longer wire, the potential gradient (K = V/L) decreases for the same potential drop (V). A smaller potential gradient means a larger change in balancing length for a given small change in EMF, making the measurement more precise and the instrument more sensitive.
7. What is the importance of the 'potential gradient' in the working of a potentiometer?
The 'potential gradient' (K) is the most critical parameter in a potentiometer's operation. It represents the uniform fall of potential per unit length of the wire (measured in V/m or V/cm). Once a constant current is set in the primary circuit, K remains constant. The unknown EMF (E) of any cell can then be determined directly by finding the balancing length (l) where the galvanometer shows no deflection, using the simple formula E = K × l. A smaller, constant potential gradient is key to an accurate and reliable measurement.
8. What would happen if the EMF of the driver cell in the primary circuit is less than the EMF of the cell being measured?
If the EMF of the driver cell is less than the EMF of the cell in the secondary circuit, it will be impossible to find a null point. The galvanometer will show a deflection in only one direction, regardless of where the jockey is placed on the potentiometer wire. This occurs because the maximum potential drop that can be provided across the entire potentiometer wire is less than the EMF it is trying to balance, so a balance condition can never be achieved.
