How is a galvanometer converted into a voltmeter?

Short Answer:

A galvanometer is converted into a voltmeter by connecting a high-value resistor in series with it. This allows the galvanometer to measure higher voltages without being damaged, as the series resistor limits the current passing through the coil.

The value of the series resistor is carefully selected so that the combined device (galvanometer + resistor) can measure a specific voltage range accurately. This simple modification enables the sensitive galvanometer to function safely and effectively as a voltmeter for DC voltage measurements.

Detailed Explanation:

Galvanometer conversion into voltmeter

A galvanometer is a sensitive instrument designed to detect and measure very small electric currents. However, it is not suitable for directly measuring high voltages because its coil can only tolerate small currents. To convert it into a voltmeter, which can measure larger potential differences, we must limit the current entering the galvanometer using a series resistor.

This transformation allows the galvanometer to safely measure voltages across electrical components without getting damaged or giving incorrect readings.

Working Principle:

The basic idea is to use Ohm’s Law (V = IR). By increasing the total resistance in the path of the current (through the series resistor), we can increase the voltage that causes full-scale deflection of the galvanometer without increasing the current beyond its safe limit.

Step-by-Step Conversion Process:

  1. Know Galvanometer Specs:
    • Determine the full-scale deflection current (IgI_gIg​) and the internal resistance (RgR_gRg​) of the galvanometer.
    • For example, suppose the galvanometer deflects fully at Ig=1 mAI_g = 1 \, \text{mA}Ig​=1mA and has an internal resistance Rg=100 ΩR_g = 100 \, \OmegaRg​=100Ω.
  2. Decide Desired Voltage Range:
    • Suppose we want the voltmeter to measure up to 5 volts.
  3. Calculate Series Resistance (R):
    • Using Ohm’s Law:

V=Ig⋅(R+Rg)V = I_g \cdot (R + R_g)V=Ig​⋅(R+Rg​) R=VIg−RgR = \frac{V}{I_g} – R_gR=Ig​V​−Rg​ R=50.001−100=5000−100=4900 ΩR = \frac{5}{0.001} – 100 = 5000 – 100 = 4900 \, \OmegaR=0.0015​−100=5000−100=4900Ω

  1. Connect in Series:
    • Connect a 4900-ohm resistor in series with the galvanometer.
    • Now, this combination can be used as a voltmeter that measures 0–5V.

Important Points:

  • The series resistor is called a multiplier resistor.
  • The higher the voltage range, the larger the resistor needed.
  • The scale of the galvanometer needs to be calibrated in volts after conversion.
  • Only DC voltages can be measured unless additional circuitry is used for AC.

Advantages of Conversion:

  • Economical way to create voltmeters for different ranges.
  • Makes use of existing galvanometer devices.
  • Useful in laboratory and educational experiments.

Limitations:

  • Only suitable for DC voltage measurements.
  • Accuracy depends on the precision of the series resistor.
  • Cannot handle very high voltages without specialized components.
Conclusion:

A galvanometer is converted into a voltmeter by simply adding a high resistance in series, which allows it to measure higher voltages by limiting the current through its delicate coil. This method is based on Ohm’s law and ensures that the galvanometer is protected from overload. Once calibrated, this setup provides an accurate and cost-effective way to measure DC voltages across various electrical components.