Question

An ammeter with input resistance of 50 $\Omega$ gives full scale deflection for 50 $\mu$A current.
The input resistance of a 0–1 mA ammeter obtained by connecting a shunt across the 0–50 $\mu$A meter will be:

• A. 50 $\Omega$
• B. 25 $\Omega$
• C. 5 $\Omega$
• D. 2.5 $\Omega$

Hint:

To extend ammeter range using a shunt, calculate total input resistance by dividing voltage across the meter by total current.

Answer 1

The Correct Option is D

To increase the range of the ammeter to 1 mA, a shunt resistor is connected in parallel to the existing meter.
The total current is 1 mA, out of which 50 $\mu$A flows through the meter and the remaining 950 $\mu$A through the shunt.
Let $R_s$ be the shunt resistance. Since both resistances are in parallel and carry respective currents, the voltage across them is equal.
\[ V = 50 \times 10^{-6} \times 50 = 2.5 \text{ mV} \] \[ R_{\text{total}} = \frac{V}{I_{\text{total}}} = \frac{2.5 \times 10^{-3}}{1 \times 10^{-3}} = 2.5 \, \Omega \]