Question

A moving-coil instrument gives full-scale deflection for 1 mA and has a resistance of $5\ \Omega$. If a resistance of $0.55\ \Omega$ is connected in parallel to the instrument, what is the maximum value of current it can measure?

• A. 5 mA
• B. 10 mA
• C. 50 mA
• D. 100 mA

Hint:

Use the formula: \textbf{\(I = I_g (1 + \frac{R_g}{R_s})\)} when a shunt is used to convert a galvanometer into an ammeter.

Answer 1

The Correct Option is B

The moving-coil instrument acts as a basic galvanometer with internal resistance $R_g = 5\ \Omega$ and full-scale deflection current $I_g = 1\ \text{mA}$.
A shunt resistor $R_s = 0.55\ \Omega$ is connected in parallel to extend the range.
The maximum current $I$ it can now measure is given by:
\[ I = I_g \left(1 + \frac{R_g}{R_s} \right) \]
Substituting values:
\[ I = 1\ \text{mA} \left(1 + \frac{5}{0.55} \right) = 1\ \text{mA} \left(1 + 9.09 \right) = 10.09\ \text{mA} \]
Approximately, the instrument can now measure up to 10 mA.