What is potentiometer?

Short Answer

A potentiometer is a device used to measure the voltage (potential difference) of a circuit accurately. It works on the principle of comparing an unknown voltage with a known reference voltage. It consists of a long uniform wire, a sliding contact, and a power source.

Since the potentiometer uses a null method (no current flows through the unknown source during measurement), it provides very accurate results. It is commonly used to measure EMF of cells, compare voltages, and determine internal resistance of batteries.

Detailed Explanation

Potentiometer

A potentiometer is a simple electrical instrument used to measure potential difference or electromotive force (EMF) with great accuracy. It is widely used in physics laboratories because it does not draw any current from the source being measured. This is the main reason why the potentiometer gives more accurate results than a voltmeter. A voltmeter always draws a small current from the circuit, which can slightly change the actual voltage, but a potentiometer avoids this problem by using a null balance method.

A potentiometer typically consists of a long uniform resistance wire made of materials like manganin or constantan. A sliding contact (jockey) moves along the wire, allowing different lengths of the wire to be used for comparison with the unknown voltage.

Working principle of a potentiometer

The potentiometer works on the principle that:

The potential difference across a uniform wire is directly proportional to its length.

This means:

V ∝ L

Where:

  • V = potential difference
  • L = length of the wire

When a steady current flows through the long uniform wire, each centimeter of the wire has the same potential drop. By selecting different lengths of the wire, we get different voltages.

Construction of a potentiometer

A laboratory potentiometer typically contains:

  1. A long uniform resistance wire
    • Usually 1 metre, 2 metres, or 10 metres long
    • Mounted on a wooden or metallic board
  2. A rheostat
    • Controls the current flowing through the wire
  3. A standard cell or battery
    • Provides a constant voltage across the wire
  4. A sliding contact (jockey)
    • Moves along the wire to measure different voltages
  5. A galvanometer
    • Detects whether current is flowing or not
  6. A key or switch
    • Used to include or exclude the circuit

This arrangement makes the potentiometer simple yet powerful for voltage comparisons.

How a potentiometer works

  1. A constant current is passed through the long resistance wire.
  2. This creates a uniform potential gradient along the wire.
  3. The unknown voltage source (cell or battery) is connected to the potentiometer.
  4. The sliding jockey is moved along the wire.
  5. At a certain length, the galvanometer shows zero deflection.
  6. This means the voltage across that length of wire is equal to the unknown voltage.
  7. Using the length and the known potential gradient, the unknown voltage is calculated.

This method is extremely accurate because at the null point, there is no current drawn from the unknown source, and hence no disturbance to its voltage.

Uses of potentiometer

The potentiometer has several important applications:

  1. Measurement of EMF of a cell

The potentiometer can measure EMF of cells more accurately than a voltmeter.

  1. Comparison of EMFs of two cells

Two cells can be compared by finding the balancing lengths for each one.

  1. Measurement of internal resistance of a cell

When a cell delivers current through a resistor, its voltage drops. This drop and EMF can be used to calculate internal resistance.

  1. Measurement of unknown voltage

By comparing with a standard cell, the potentiometer can measure unknown voltages in circuits.

  1. Calibration of voltmeters

Voltmeter accuracy can be checked and corrected using a potentiometer.

Advantages of potentiometer

  • Very accurate because it uses a null method
  • No current drawn from the source being measured
  • Not affected by internal resistance of the cell
  • Can measure very small voltages precisely
  • Useful for calibration and scientific experiments

Limitations of potentiometer

  • Requires a steady current for accurate readings
  • Takes time to set up and balance
  • Needs long and uniform resistance wire
  • Not suitable for fast-changing voltages

Despite these limitations, potentiometers remain one of the most reliable instruments for voltage measurement.

Comparison with voltmeter (explained verbally)

  • A voltmeter draws some current → less accurate
  • A potentiometer draws zero current → highly accurate
  • Potentiometer is used for precise measurement
  • Voltmeter is used for quick, approximate readings

This explains why laboratories prefer potentiometers for EMF and internal resistance experiments.

Conclusion

A potentiometer is a device used to measure potential difference or EMF by comparing an unknown voltage with a known voltage using a null method. It is highly accurate because it draws no current from the source being measured. With its simple construction and precise working principle, the potentiometer is widely used for measuring EMF, comparing voltages, and determining internal resistance. It is an essential instrument in physics laboratories and electrical measurements.