State the principle of working of a galvanometer.
A galvanometer of resistance G is converted into a voltmeter to measure upto V volts by connecting a resistance R1 in series with the coil. If a resistance R2 is connected in series with it, then it can measure upto V/2 volts. Find the resistance, in terms of R1 and R2, required to be connected to convert it into a voltmeter that can read upto 2 V. Also find the resistance G of the galvanometer in terms of R1 and R2.Principle:
When a current carrying loop or coil is placed in the uniform magnetic field, moving coil galvanometer experiences a torque.
A high resistance is connected in series with the galvanometer to convert it into a voltmeter.
The value of resistance is given by, 
where,
V is the potential difference across the terminals of the voltmeter. 
is current through galvanometer and
G is the resistance of the galvanometer.
When resistance R1 is connected in series with the galvanometer, 
When resistance R2 is connected in series with the galvanometer, 
... (2)
From equations (1) and (2), we get 
Resistance R3 required to convert galvanometer into voltmeter of range 0 to 2V is given by, 
Therefore, R1 - 2R2 is the galvanometer resistance in terms of R1 and R2
