To convert the given galvanometer of known resistance and figure of merit into a voltmeter of desired range and to verify the same

 CONVERSION OF GALVANOMETER INTO VOLTMETER

A galvanometer is an electronic device that is used to detect mild currents in an electrical circuit. Galvanometer works on the principle of a coil placed in a uniform magnetic field that experiences a torque when an electric current is set up in it  We can convert a galvanometer into a voltmeter by connecting a large resistance in series to the galvanometer. 

Aim of the experiment :

To convert the given galvanometer into a voltmeter of the required range (0 - 3V) and verify the same.

Apparatus required :

1. Connecting wire
2. Galvanometer
3. Battery
4. Voltmeter
5. Resistance box
6. Plug Key
7. Rheostat

Principle of the experiment :

Galvanometer can be converted into a voltmeter by connecting suitable high resistance in series with it so that a very small current flows through the galvanometer

Formula :

01. Current (in Ampere) required for full-scale deflection in galvanometer, Ig = N*K

Where,

            N - Number of divisions on either side of zero of the galvanometer.

            K - Figure of merit of galvanometer.

02. High resistance to be connected in series with the galvanometer, R = V / Ig - G

Where,

            G - Resistance of the Galvanometer.

            V - Maximum voltage to be measured.

Circuit diagram :

The circuit diagram for converting the given galvanometer of known resistance and figure of merit into a voltmeter is as follows below :

Conversion of Galvanometer into voltmeter

Procedure :

• The value of resistance is calculated using the formula, R = V / Ig - G

• The circuit connections are made as shown in the circuit diagram.

• The calculated high resistance, R is unplugged in the standard resistance box and the rheostat is adjusted so that the voltage shown in the voltmeter is equal to the desired range (say 3V).

• Resistance from the resistance box is adjusted such that the galvanometer shows full-scale deflection. Then R' from the resistance box is noted.

• P' is equal or nearly equal to R.  Hence conversion is verified.

Observation :

1. Resistance of the galvanometer, G (given) = ..................................Ω ohm.
2. Figure of merit of the galvanometer, K (given) = ..................................A/div.
3. Number of divisions on either side of zero of the galvanometer scale, N = ..................................A.
4. Current required for producing full-scale deflection of N divisions, Ig = N*K = ..................................Ampere
5. Maximum voltage to be measured, V (say 3V) = ..................................V.

Calculation :

High resistance to be connected in series with the galvanometer, R = V / Ig - G

Result :

01. The value of the theoretically calculated value of series resistance, R = ..................................Ω ohm.

02. Practically observed value of series resistance, R' = ..................................Ω ohm.

Note :

High resistance of order of 10K should be unplugged in the resistance box first and then key has to be closed to avoid any damage to the galvanometer.