#### Question

Explain how moving coil galvanometer is converted into a voltmeter. Derive the necessary formula.

#### Solution

a. To use a M.C.G as a voltmeter, its resistance should be increased to a desired value and

an arrangement should be provided to measure large potential difference. This is

achieved by connecting a high resistance in series with the M.C.G.

b. Let ‘G’ be the resistance of the galvanometer coil and ‘Ig’ be the maximum current

which can be passed through the galvanometer coil for full-scale deflection.

c. Let ‘V’ be the potential difference to be measured.

Let ‘Rs’ be the resistance connected in series with the galvanometer coil.

d. From Ohm’s law,

V = I_{g} (G + R_{s})

G + R_{s}=`V/I_g`

R_{s}=`V/I_g-G`

Knowing V, I_{g} and G, value of R_{s} can be determined.