Question

A moving coil galvanometer of resistance \(100\,\Omega\) shows a full scale deflection for a current of \(1\,\text{mA}\). The value of resistance required to convert this galvanometer into an ammeter, showing full scale deflection for a current of \(5\,\text{mA}\), is ________ \(\Omega\).

• A. 25
• B. 2.5
• C. 10
• D. 0.5

Hint:

To convert a galvanometer into an ammeter, always connect a low resistance shunt in parallel.

Answer 1

The Correct Option is A

To convert a moving coil galvanometer to an ammeter showing full-scale deflection for a higher current, we need to add a shunt resistance (Rs) in parallel with the galvanometer. The galvanometer, in this case, acts as a current-detecting component.

Given data:

• Resistance of galvanometer, \( R_g = 100 \, \Omega \)
• Full-scale deflection current for galvanometer, \( I_g = 1 \, \text{mA} = 0.001 \, \text{A} \)
• Desired full-scale deflection current for ammeter, \( I = 5 \, \text{mA} = 0.005 \, \text{A} \)

We need to calculate the shunt resistance \( R_s \). The formula to calculate the shunt resistance is:

\(R_s = \frac{R_g \cdot I_g}{I - I_g}\)

Substituting the given values:

• \(R_s = \frac{100 \times 0.001}{0.005 - 0.001}\)
• \(R_s = \frac{0.1}{0.004}\)
• \(R_s = 25 \, \Omega\)

Therefore, the value of resistance required to convert the galvanometer into an ammeter showing full-scale deflection for a current of \(5\,\text{mA}\) is \(25\,\Omega\).

Thus, the correct answer is 25 \(\Omega\).