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Question
Explain how moving coil galvanometer is converted into a voltmeter. Derive the necessary formula.
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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 = Ig (G + Rs)
G + Rs=`V/I_g`
Rs=`V/I_g-G`
Knowing V, Ig and G, value of Rs can be determined.
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