Question

Resistance of the voltmeter is 10000 $\Omega$. To increase range of voltmeter from 25 V to 250 V, resistance connected in series is

• A. 900 $\Omega$
• B. 9000 $\Omega$
• C. 90000 $\Omega$
• D. 90 $\Omega$

Answer 1

Answer: (C)

90000 $\Omega$

Answer 2

The full-scale current $I$ of the voltmeter is:

$$I = \frac{V_{\text{meter}}}{R_{\text{meter}}} = \frac{25\,\text{V}}{10000\,\Omega} = 0.0025\,\text{A}$$

To extend the range to 250 V, the extra voltage drop required across the series resistor is:

$$V_{\text{series}} = 250\,\text{V} - 25\,\text{V} = 225\,\text{V}$$

Thus the series resistance is calculated as:

$$R_{\text{series}} = \frac{V_{\text{series}}}{I} = \frac{225\,\text{V}}{0.0025\,\text{A}} = 90000\,\Omega$$