Page 31 - Physics - XII

Apparatus and Materials Required
A weston type galvanometer of known resistance and figure of merit, a battery, a rheostat, a one-
way key, a resistance box of the range 0 to 10,000 Ω, a voltmeter of 3 V range, connecting wires, and sand
paper

Principle/Theory
A galvanometer can be converted into a voltmeter of I G R
desired range by connecting a suitable high resistance R g
in series with the galvanometer. Let G is the resistance of
the galvanometer which gives full scale defl ection when I g
current fl ows through it. Let V is the range of the voltmeter, V
the series resistance R required for the conversion is given Fig. 5 (a). 1: Conversion of a galvanometer
by into a voltmeter
V
R = − G
I g

Procedure
1. Note the given value of galvanometer resistance G and its fi gure of merit K. Also, count the total
number of divisions n on either side of the zero of the galvanometer scale.
2. Find the value of current I for full scale defl ection in the galvanometer with the help of the relation,
g
I = nK.
g
3. Calculate the value of required series resistance R to be connected in series with the galvanometer for
the conversion of the given galvanometer into voltmeter of the given range (say 3 V) by the relation,
V
R G.
I g
4. Connect the resistance box of range 0 to 10,000 Ω in series with the galvanometer and take out the
plugs of resistance R equal to the value of resistance as calculated in step 3 from it. Now, galvanometer
with this resistance in series is converted into voltmeter of the range 0 to 3V.

5. For verifi cation of the converted voltmeter, make the connections shown in the circuit diagram
(Fig. 5(a).2).
6. Take out the plugs of calculated resistance R from the Battery Key
resistance box of the range 0 to 10,000 Ω, close the + – K
key K, and adjust the rheostat so that the defl ection in Rheostat

galvanometer becomes maximum.
7. Note the readings of standard voltmeter and galvanometer. Voltmeter –
+
Convert the galvanometer reading into volt and fi nd the
diff erence between them, if any. This diff erence gives the
error. + G – R Converted voltmeter
8. Take four more observations by changing the resistance
with the help of the rheostat over the whole range of the Fig. 5 (a). 2: A converted galvanometer
into voltmeter and its

voltmeter. verifi cation by comparing
9. Note all the observations in the observation table. it with standard voltmeter

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