Class 12 > Unit # 18:Magnetic Fields > Conversion of Galvanometer into Ammeter & Voltmeter

Talha's Physics Academy

Electrical Instruments - Ammeters and Voltmeters

Define Ammeter. How galvanometer is converted into ammeter? Derive an expression for shunt resistance.

AMMETER

"An Ammeter is an electrical instrument used to measure electric current in amperes. This is basically a modified galvanometer."

Figure: Conversion of a galvanometer into an ammeter by connecting a parallel low shunt resistance ($R_s$).

The portion of the galvanometer whose motion causes the needle to move across the scale is called the meter-movement. Most meter-movements are very sensitive, giving full-scale deflection with a current of only a few milliamperes. Therefore, an ordinary galvanometer cannot be used directly for measuring large currents without proper modification.

CONVERSION OF GALVANOMETER INTO AMMETER

Suppose we have a galvanometer whose coil has a resistance $R_g$ and which gives full-scale deflection when a current $I_g$ passes through it. From Ohm's law, the potential difference across the galvanometer is:

$V_g = I_g R_g$

To convert this galvanometer into an ammeter capable of measuring a much larger maximum current $I$, a very low-value bypass resistor called a shunt ($R_s$) is connected in parallel with the galvanometer. The shunt diverts the excess current ($I - I_g$), ensuring that only the safe current $I_g$ passes through the galvanometer.

EXPRESSION FOR SHUNT RESISTANCE ($R_s$)

Since the meter-movement and the shunt resistance are connected in parallel, the potential difference across the galvanometer equals the potential difference across the shunt:

$V_{\text{galvanometer}} = V_{\text{shunt}}$

$I_g R_g = (I - I_g) R_s$

Rearranging the equation to solve for the shunt resistance $R_s$:

$R_s = \frac{I_g R_g}{I - I_g} \quad \text{(Proved)}$

Define Voltmeter. How galvanometer is converted into voltmeter? Derive an expression for series high resistance.

VOLTMETER

"A voltmeter is an electrical device which measures the potential difference in volts between two points in a circuit."

Figure: Conversion of a galvanometer into a voltmeter by connecting a high series resistance ($R_H$).

A voltmeter is always connected in parallel across the two points where potential difference is to be measured. Its resistance must be extremely high compared to the circuit resistance; otherwise, it will draw significant current, load the circuit, and alter the potential difference being measured.

CONVERSION OF GALVANOMETER INTO VOLTMETER

To ensure a very high resistance, a very large resistance $R_H$ is connected in series with the galvanometer meter-movement.

EXPRESSION FOR SERIES RESISTANCE ($R_H$)

Suppose a galvanometer has a coil resistance $R_g$ and gives full-scale deflection with current $I_g$. To make a voltmeter with a voltage range of $V$ volts, the high resistance $R_H$ in series must limit the total current to $I_g$ when connected across $V$ volts.

Applying Ohm's law to the entire series combination:

$V = I_g (R_g + R_H)$

Dividing both sides by $I_g$:

$\frac{V}{I_g} = R_g + R_H$

Rearranging to solve for the high series resistance $R_H$:

$R_H = \frac{V}{I_g} - R_g \quad \text{(Proved)}$

© 2026 Talha's Physics Academy. All rights reserved.