Potentiometer: Working Principle, Types & Applications

Potentiometer is a three- terminal resistor used for measuring the unknown voltage by comparing it with the known voltage. It is used to compare the electromotive force of two cells, and to measure the potential difference between two points of a given circuit and the internal resistance of a cell. The resistance is manually changed to control the current flow. The work of potentiometers depends on varying the position of a sliding contact across a uniform resistance. The potentiometer also finds application as a voltage divider. 

Read More: Current Electricity

Key Terms: Potentiometer, Voltage, Resistor, Potential Difference, Resistance, Length, Rotary, Linear, Voltage divider, Current flows, Internal resistance, Electromotive force

What is a Potentiometer?

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A potentiometer or a ‘pot’ is a mechanically operated three-terminal analog resistor in which two terminals are fixed and the remaining one is a variable voltage divider. The advantage of a potentiometer is that it doesn’t draw electric current from the voltage source and this is done by the resistance. The known voltage is drawn by any supply source. Some important features are:

• Used to measure potential difference in a circuit
• The potential difference is the amount of work that is done to bring a charge from the first point to the second point in a circuit.
• In the presence of a potential difference in a circuit, the current flows through it. 
• Potentiometer is used in scenarios where high accuracy is needed and it uses comparative method to provide accurate results. 

Potentiometer Terminals

The video below explains this:

Potentiometer Detailed Video Explanation:

Common Terminologies for Potentiometer

• Slider Pot / Slide Pot- can be adjusted with a wiper by sliding on either sides with a thumb or a finger
• Thumb Pot / Thumbwheel Pot- can be adjusted using a small rotating potentiometer called a thumbwheel
• Trimmer Pot / Trim Pot- can be adjusted single time for fine-tuning of electrical signals

Symbolic Representation of Potentiometer

Two symbolic representations are very commonly used for potentiometers circuit diagrams- the American Standard and the International Standard.

• American Standard- comprises of zig-zag lines with three terminals between the straight lines. 
• InInternational Standard- comprises of a rectangular box with three terminals between the two straight lines. 

Potentiometer Symbolic Representation

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Potentiometer Working Principle

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A potentiometer works on the basic principle that the potential across a wire is directly proportional to its length. The current flow is zero when there is no potential difference between any two nodes. Let us understand the potentiometer working in a little detail. 

• The potentiometer contains a long resistive wire (L) and a battery with driver cell voltage (EMF).
• Two batteries with same EMF are connected to a galvanometer
• Positive terminals are connected to a galvanometer and negative terminals are connected together
• The galvanometer shows null deflection due to the absence of current flow as batteries have same EMF

Potentiometric wires are long and made with constantan and manganin and a battery of known emf. Following is a simple circuit diagram to understand the potentiometer working better.

Potentiometer Working Principle

The electric circuit consists of:

• Galvanometer which detects current and measures its magnitude
• Ammeter for measuring the value of electric current in the circuit
• Jockey provides the metal contact on the wire potentiometer
• Wire that is made of constantan and manganin due to its high specific resistance and low temperature coefficient
• Rheostat that controls the current and varies the resistance without interruptions

Also Read: Resistors in Parallel Formula

Construction of Potentiometer

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The working principle of a potentiometer depends on any potential segment of the wire which is directly proportional to the length of the wire having constant current flow and a uniform cross-sectional area. The concept of the potentiometer working principle can be understood as:

Potentiometer Construction

As per Ohm’s law: V = IR

Where, V is Voltage,  I is Current & R is the total resistance

Similarly, 

R = ρL/A 

 V = I*(ρL/A)

Where,  ρ is Resistivity and A is Cross-sectional Zone

Now, ρ and A are constant, and I is also constant for a rheostat.

Lρ/A = K

V = KL

Therefore, 

E = Lρx/A = Kx 

Where,

E is a Cell with Lower EMF, x is the Length of the Potentiometer Wire, and K is Consistent 

As the potential difference is equivalent to zero and there is no flow of current it implicates that the galvanometer G has null detection. So, the length of x is the null point.

After knowing x and K, the unknown electromotive force (EMF) can be found. Because EMF has two cells, let L₁ be the null point length of the initial cell with EMF E₁ and L₂ be the null point length of the second cell with EMF E₂. Then, 

E₁/E₂ = L₁/L₂

To Determine the EMF of the cell

Let I be the current flowing through the wire, then we have

I = ɛ/(r+R)

Where ɛ is the emf of the cell in the primary circuit

r is the internal resistance

R is the resistance of the wire

Voltage across potentiometer wire, V_AB = V₀

Potential gradient can be understood as the fall of the potential per unit length of the wire of the potentiometer.

Potential Gradient, Z = (V₀/L)

The point on the wire when the galvanometer does not show any deflection is called the null point. The length of the wire AP is taken as ‘l’.

Potential difference between A and P – \(\begin{array}{l}V_{A}-V_{P}=\frac{\frac{\epsilon}{(r+R)}R}{L}.l\end{array}\)

V_A – V_P = (V₀/L)l

Zl = Ɛ’ (as V₀ = iR)

Ɛ’ is the emf of the cell that is connected in the secondary circuit

Read more: Current Density Formula

Measuring Internal Resistance of a Cell

The potentiometer is also used to measure the internal resistance of the cell. To determine the internal resistance of a cell, it is connected across the resistance box through the key. When the key is open the null point is determined at a distance l₁ from A. Then,

ε = Zl₁ — (1)

When the key is closed a current denoted by I is passed through the resistance box. Let the null point now be at a distance of l₂ from point A. Therefore, the potential difference will be

v = Zl₂ — (2)

From (1) and (2) we get

ε/v = l₁/l₂ — (3)

We know ε = i(r+R) and v = iR

Therefore, [i(r+R)/ iR] = l₁/l₂
Internal resistance r can be expressed as,

 \(\begin{array}{l}r= R\left ( \frac{l_{1}}{l_{2}}-1\right )\end{array}\)

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Types of Potentiometer

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The potentiometer is classified into three broad types based on the wiper movement:

• Rotary Potentiometer
• Linear Potentiometer 
• Digital Potentiometer

Potentiometer Classification

Let us look at each of these and their subtypes in a little detail below.

Rotary Potentiometer

• The wiper of the potentiometer moves along a circular path. 
• Has two terminal contacts with a uniform resistance placed in between in a semi-circular pattern.
• Used for getting adjustable voltage supply to circuits and equipment.
• Example: Volume controller of radio transistor whose rotary knob controls the amplifier supply.

Rotary Potentiometer

Rotary Potentiometer is further classified into the following categories:

1. Single turn Pot: A single rotation of the wiper is around 270 degrees or 3/4 of a complete turn.
2. Muti turn Pot: Multiple rotations (mostly 5, 10, 15 or so) can be taken by potentiometers if a wiper is in either spiral or helix form or else, by using a worm-gear.
3. Concentric Pot: Two potentiometers are adjusted individually by using concentric shafts to have two controls on one unit.
4. Servo Pot: A motorized pot used to control automatically or adjust a servo motor.

Linear Potentiometer

• The sliding contact gets moved linearly unlike the rotary potentiometer.
• It is a type of a position sensor.
• Measures distance along a single axis (up and down or left and right).
• Two ends of the straight resistors are connected across the source voltage.
• Sliding contact slides on the resistor through a track.

Linear Potentiometer

Linear Potentiometer is further classified into the following categories:

1. Slide Pot: Also called the fader, is a single linear slider potentiometer constructed with conductive plastic.
2. Muti turn Slide: Constructed with a spindle that activates the linear potentiometer wiper.
3. Dual Slide Pot: Used to control two parallel potentiometers: a dual slide and a single slider.
4. Motorized Fader: Automatically controlled by a servo motor.

Digital Potentiometer

• There are two fixed end terminals and one wiper which varies the wiper voltage.
• Used for manual controls like calibrating, controlling brightness and sound.
• They are compact and require less space.
• There is no wiper contamination, mechanical wearing off, nil sensitivity to vibrations and no impact due to humidity.

Digital Potentiometer

Read More: Cells in Series and Parallel

Applications of Potentiometer

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Potentiometers act as a voltage divider which is why it is used in different and a wide range of applications, some of the applications are listed below:

• Audio control: For controlling or adjusting the loudness and other audio-related signals both rotary and linear potentiometers are used.
• Television: The factors in television that are controlled or adjusted by these potentiometers are picture brightness, contrast, color response, etc.
• Motion control: Potentiometers are used as a servomechanism (a position feedback device) to create a closed-loop control.
• Transducers: Transducers allow large output signals so potentiometers find applications in designing of displacement transducers.

Applications of Potentiometer

Discover about the Chapter video:

Current Electricity Detailed Video Explanation:

Read More: Relation Between articles (Physics)

Things to Remember

• The potentiometer is a three-terminal resistor with a sliding contact that acts as a variable voltage divider.
• They can be used as dimmers to control the brightness of light.
• Potentiometer works on the principle that the potential difference across the wire is directly proportional to the wire length. 
• Potentiometer is divided into rotary, linear, and digital types.
• The potentiometer can be used to compare the EMF of 2 cells and measure the resistance of a cell.
• It is also used to determine the EMF of a cell.
• The sensitivity of the potentiometer is the change in the potential difference that can be measured by a potentiometer. 

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Previous Year Questions 

1. If now we have to change the null point at 9^th  wire, what should we do?… [DUET 2007]
2. The electrical permittivity and magnetic permeability of free space are​… [DUET 2003]
3. Just after key K is pressed to complete the circuit, the reading will be​ …. [KEAM 1999]
4. The resistance between any two terminals is when connected in a triangle is…. [NEET 1993]
5. potential drop through 4Ω  resistor is… [NEET 1993]
6. The potential difference per unit length of the wire will be… [NEET 1999]
7. Value of R for which the power delivered in it is maximum is given by... [NEET 1992]
8. Three resistances each of 4Ω are connected to form a triangle… [NEET 1993]
9.  In a potentiometer the null point is received at 7^th wire… [DUET 2007]
10. A 2V battery, a 15Ω resistor and a potentiometer of 100 cm length… [JCECE 2011]
11. A potentiometer wire  AB having length L and resistance 12r is joined to… [JEE Main 2019]
12. A resistance is shown in the figure. Its value and tolerance are given respectively by ...[JEE Main 2019]
13. The resistance between any two vertices of the triangle is...[JEE Main 2019]
14. In The Given Circuit Diagram…...[JEE Main 2019]
15. In the given circuit, an ideal voltmeter connected across the 10Ω resistance...[JEE Main 2019]
16. In the experimental set up of meter bridge shown…..[JEE Main 2019]
17. In a potentiometer experiment, when three cells A,B, and C  are connected in series…..[KEAM]