Question

A moving coil meter of $50\Omega$ resistance reads up to 25 mA. What is the value of series resistance, so that it can be read up to 10 V?

• A. 300 $\Omega$
• B. 200 $\Omega$
• C. 400 $\Omega$
• D. 350 $\Omega$

Hint:

Always remember: To make a Voltmeter, add Very high resistance in Series. To make an Ammeter, add a tiny resistance in Parallel (Shunt).

Answer 1

The Correct Option is D

To convert a galvanometer (moving coil meter) into a voltmeter capable of measuring a higher voltage, a high resistance (called a multiplier) must be connected in series with the meter. 1. Given Data:

• Meter Resistance ($R_m$) = $50 \Omega$

• Full-scale deflection current ($I_m$) = 25 mA = 0.025 A

• Desired Full-scale Voltage ($V$) = 10 V

2. Formula for Series Multiplier Resistance ($R_s$): The total resistance required to limit the current to 25 mA when 10 V is applied is: $$R_{total} = \frac{V}{I_m}$$ $$R_{total} = \frac{10}{0.025} = 400 \Omega$$

3. Calculating the Multiplier ($R_s$): Since the total resistance is the sum of the meter resistance and the series resistance: $$R_{total} = R_m + R_s$$ $$400 = 50 + R_s$$ $$R_s = 400 - 50 = 350 \Omega$$ Thus, a series resistance of $350 \Omega$ is needed.