Page 33 - Physics - XII

EXPERIMENT - 5(b)

Aim

To convert the given galvanometer (of known resistance and fi gure of merit) into an ammeter of desired
range and to verify the same.

Apparatus and Materials Required
A weston type galvanometer of known resistance and fi gure of merit, a battery, a one-way key, a rheostat, an
ammeter of given range, a screw gauge, a constantan or manganin wire, connecting wires, sand paper
and a wire cutter

Principle/Theory
Let G be the galvanometer resistance and S be the calculated I Galvanometer
value of the shunt resistance. I is the current fl owing through g
g
the galvanometer which gives full scale defl ection and I is the I G
total current fl owing in the circuit for full scale defl ection, (I–Ig)
(I – I ) current is passing through the shunt S, as shown in
g
Fig. 5(b).1. S
We have, Shunt resistance
I G
S = g …(1) Fig. 5(b).1: Conversion of galvanometer
I I g into an ammeter

Therefore, to convert a galvanometer which shows full scale defl ection for the current I into an ammeter
g
of range 0 to I ampere, the value of the required shunt resistance is given by eqn. (1). This shunt resistance
S is made by using a wire of uniform area of cross-section, whose specifi c resistance ρ is known and its
length l can be calculated by the relation

l
S =
r 2
2
rS
or l = …(2)

where r is the radius of the wire.

Procedure
1. Note the given value of the resistance of the galvanometer G and its fi gure of merit K.

2. Count the total number of divisions on either side of the zero of the galvanometer scale.
3. By using the relation I = K.n, calculate the value of I current for which galvanometer gives full scale
defl ection. g g
4. Calculate the value of shunt resistance S by substituting the values of G, I and range of the ammeter
I G g
I, in the formula, S g
I I g
5. Take a wire available in the laboratory say copper wire and fi nd its average corrected diameter using a

screw gauge and calculate the average corrected radius of the wire.

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