To determine the resistance of a galvanometer by half deflection method and to find its figure of merit of galvanometer

FIGURE OF MERIT OF GALVANOMETER

The figure of merit of a galvanometer is the current required to generate a unit deflection in the galvanometer by changing resistance R in the resistance box. The galvanometer is the instrument that is used to measure the intensity and to show the direction of the current in the electrical circuit.

Aim of the experiment :

Determination of the resistance of the galvanometer by half deflection method and to find its figure of merit of galvanometer.

Apparatus required :

1. Battery
2. Resistance box of different resistances
3. Galvanometer
4. Connecting wire
5. Plug Keys

Principle of the experiment :

Deflection in a galvanometer is directly proportional to the current through the galvanometer.

I =  K * θ

Where,

            K - Figure of merit of galvanometer

Formula :

01. Figure of merit of galvanometer, (in A div^-1), K = E/(R+G)θ

Where,

            E - EMF of the cell

            R - Resistance in series with the galvanometer

            G - Galvanometer resistance

            θ - Deflection in the Galvanometer    

Circuit diagram :

The circuit diagram for resistance of a galvanometer by half deflection method is shown below :

The resistance of a galvanometer by half deflection method

Procedure :

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

• Key K2 is opened and keyis  K1 closed. the suitable resistance R is unplugged to get an even deflection θ in the galvanometer. 

• K2 is closed and suitable resistance S is unplugged so that deflection becomes θ/2. then the resistance of the galvanometer G is calculated using the formula, G = R*S/R-S

• The experiment is repeated for different values of θ and the average value of G is found.  
• The EMF E of the cell is measured using a voltmeter.

• The figure of merit of the galvanometer is calculated in each case using the formula, K = E/(R+G)θ

• Experiment is repeated for different values of R and the value of K is found.

Observation :

Emf of the battery. E = ..................................V 

Tabular column :

Note down the observed values in the respective tabular column.

Trial No	Resistance, R in (Ω Ohm)	Deflection, θ in (div)	S for θ/2 in (Ω Ohm)	G = R*S/R-S in (Ω Ohm)	K = E/(R+G)θ (in A div -1)
01.
02.
03.
04.
05.
Average G = ...........Ω
Average K = ...........(in A div -1)

Result :

The resistance of the Galvanometer, G = ...........Ω

The figure of merit of the pointer galvanometer, K = ...........(in A div ^-1)