A voltmeter is a high resistance galvanometer. It is used to measure the potential difference between two points of circuit in volt.
A galvanometer cane converted into a Voltameter by connecting aging resistance in series with the galvanometer. The value of the resistance depends upon the range of voltmeter a can be calculated as follows.
Let G = resistance of galvanometer.
N = number of scale divisions in the galvanometer,
K = figure of merit of galvanometer current for one scale deflection of the galvanometer.
∴ Current which produces full scale deflection in the galvanometer, lg = nK.
Let V be the potential difference to be measured by galvanometer.
To do so a resistance R of such a value is connected in series with the galvanometer so that if a potential difference V is applied across the terminals A and B a current Lg flows through the galvanometer.
Noe total resistance of voltmeter = G + R
From ohm law lg = V / G + R
Or G + R = V / lg or R = V/ lg - G
If this value of R is connected in series with galvanometer is works as a voltmeter of the range O to V volt. Now the same scale of the galvanometer which was recording the maximum potential L G before conversion will record ht potential V after conversion into voltmeter. If means each division of the scale in voltmeter will show higher potential than that of the galvanometer.
Effective resistance R of converted galvanometer into voltmeter is R = G + R
For voltmeter a high resistance R is connected in series with the galvanometer therefore the resistance of voltmeter is very large as compared to that of galvanometer. The resistance of an ideal voltmeter is infinity.
A galvanometer with a coil of resistance 12 Ω showed a full scale deflection for a current of 2.5 mA. How will you convert it into a voltmeter of range 7.5 V also find the total resistance of voltmeter formed.
A galvanometer can be converted into a voltmeter to given range by connecting a suitable résistance R inn series of galvanometer, which is given by
R = V / I - G
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