Question

A moving coil galvanometer has a resistance of 15 Ω and takes 20 mA to produce full-scale deflection. How can this galvanometer be converted into a voltmeter of range 0 to 100 V?

Hint:

To convert a galvanometer into a voltmeter, a high resistance is connected in series. The value of the resistance is calculated to achieve the desired voltage range for full-scale deflection.

Answer 1

To convert the galvanometer into a voltmeter, we need to connect a series resistance \( R_s \) with the galvanometer. The value of \( R_s \) can be calculated using the following relation: \[ V = I_G (R + R_s), \] where \( V \) is the full-scale deflection voltage (100 V), \( I_G \) is the current for full-scale deflection (20 mA), and \( R \) is the resistance of the galvanometer (15 Ω). Rearranging to solve for \( R_s \): \[ R_s = \frac{V}{I_G} - R = \frac{100}{20 \times 10^{-3}} - 15 = 5000 - 15 = 4985 \, \Omega. \] Thus, the required series resistance is 4985 Ω. By connecting a 4985 Ω resistor in series with the galvanometer, it can be converted into a voltmeter with a full-scale range of 0 to 100 V.