Aim : To determine resistance of a galvanometer by half deflection method and to find its figure of merit .
Apparatus : A Weston type of galvanometer , a voltmeter , a battery /battery eliminator , two 10,000 ohm and 200 ohm ) resistance boxes , two one way key , a rheostat , a screw gauge , a meter scale , an ammeter of given range , connecting wires and a piece of sand paper .
The resistance of the given galvanometer as found by half deflection method .
G= (RxS)/(R-S) (1)
Where R is the resistance connected in series with the galvanometer and S is the shunt resistance .
The figure of merit , K =E /[(R+G)Q] (2)
Where E is the end of the cell and Q is the deflection produced with resistance R.
The maximum current that can pass through the galvanometer , Ig=nk
Where n is the total number of dimension on the galvanometer scale on either side of zero .
1. Resistance of galvanometer by half deflection method
- Make the connections accordingly as shown in circuit diagram .
- See that all plugs of the resistance boxes are tight
- Take out the high resistance (200 Ohm ) from the resistance box R and insert key K1 only .
- Adjust the value of R1 so that deflection is maximum , even in number and within the scale .
- Note the deflection . Let it be Q.
- Insert the key k2 also and without changing the value of R2 ,adjust the value of S , such that deflection in the galvanometer reduces to exactly half the value of resistance S.
- Note the value of resistance S .
- Repeat step 4 & 7 three times taking different R and adjusting .
2. Figure of merit
- Take one cell of the battery and find its emf by a voltmeter by connecting +ve of the voltmeter with +ve of cell and – ve of the voltmeter with -ve of cell . Let it be E.
- Take connection as in circuit diagram .
- Adjust the value of R to obtain a certain deflection Q when circuit is closed .
- Note the value of resistance R and deflection Q.
- Now change the value of R and note the galvanometer deflection again .
- Repeat the step with both cells of the battery with different voltages like 2,4,6 ,8 volts from battery eliminator.
- Find the figure of merit K using the formula
|Resistance (R)||Deflection (Q)||Shunt (S)||G =(RxS)/(R-S)|
|Average||G = 7.79 ohms|
|Resistance (R)||Deflection (Q)||K|
|Average||K = 0.5455 A/dn|
- G = (5.005+3+10.05+13.11)/4 [E = 1.45V]
- Figure of Merit : Current in the galvanometer per unit time
- K = IQ = [(E)/(R+G)]x[1/Q] – SI unit = AD division = 10-5 Al div
Resistance of given galvanometer is 7.79 ohm .
Figure of merit of given galvanometer is 0.545 A/dn .
All connections should be neat and tight .
The emf of cell / battery should be constant .
Source of error
The scene of the instrument may be loose .
The galvanometer division may not be of equal size.
Resistance of Galvanometer
Figure of Merit
What is the galvanometer ?
It is a device used for detecting feeble electric currents in circuit .
Define the figure of merit of a galvanometer ?
The quantity of current required to produce a deflection of one division in the galvanometer is called figure of merit of a galvanometer . It is represented by K . Its unit is ampere per division .
Why is this method called half deflection method ?
It is so because the deflection is made half by using a shunt resistance S.
Under what condition id the resistance of a galvanometer (E1) equal to shunt resistance (S)?
E1=S , only when series resistance R is very high .