Current Electricity and Electrical InstrumentHard

Question

A galvanometer has coil of resistance 100 ohm and gives a full scale defelection for 30 mA current. It is to work as a voltmeter of 30 volt range, the resistance required to be added will be

Options

A.900 Ω
B.1800 Ω
C.500 Ω
D.100 Ω

Solution

       
Here ,
Resistance of galvanometer, G = 100 Ω
Current for full scale deflection, Ig = 30mA = 30 × 10-03 A
    Ragne of voltmeter V = 30 V
To convert the galvanometer into an voltmeter of a given range, a resistance R is connected in series with it as shown in the figure
From figure
V = Ig (G + R)
or = - 100Ω
= 1000 - 100 = 900

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