Ncert Exercises & Solutions

4.27 A galvanometer coil has a resistance of 12 Ω and the metre shows full scale deflection for a current of 3 mA. How will you convert the metre into a voltmeter of range 0 to 18 V?

Resistance of the galvanometer coil, G = 12

Ω

Current for which there is full scale deflection,

I_{g} = 3 mA = 3 \times 10^{- 3} A

Range of the voltmeter is 0, which needs to be converted to 18 V.

∴ V = 18 V

Let a resistor of resistance R be connected in series with the galvanometer to convert it into a voltmeter. This resistance is given as:

R = \frac{V}{I_{g}} - G

= \frac{18}{3 \times 10^{- 3}} - 12 = 6000 - 12 = 5988 Ω

Hence, a resistor of resistance 5988

Ω

is to be connected in series with the galvanometer.

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