Question

A galvanometer has a resistance of 50 Ω and a current of 2 mA is needed for its full scale deflection. Calculate the resistance required to convert it into a voltmeter of 10 V range.

Answer 1

Given:

Resistance of Galvanometer = R_g = 50 Ω

Full scale deflection current = I_g = 2 mA  = 2 × 10⁻³ A

V = 10 V

Formula:

To convert a galvanometer into a voltmeter, a series resistance (R_s​) is connected in series with it. The series resistance is given by:

${\displaystyle R_s=\frac{V}{I_g}-R_g}$

${\displaystyle R_s=\frac{10}{2\times {10}^{-3}}-50}$

⇒ ${\displaystyle R_s=\frac{10\times {10}^3}{2}-50}$

⇒ ${\displaystyle R_s=\frac{10000}{2}-50}$

⇒ R_s = 5000 − 50

⇒ R_s = 4950 Ω

∴ The required series resistance is 4950 Ω (4.95 kΩ).