Page 25 - 12-phy-14 ELECTROMAGNETISM

14. Electromagnetism eLearn.Punjab

If we want to convert this galvanometer into an ammeter which can measure a maximum current

/, it is necessary to connect a low value bypass resistor called shunt. The shunt resistance is of
such a value that the current / for full scale deflection of the galvanometer passes through the
g
galvanometer and the remaining current (/ - / passes through the shunt (Fig 14.19).
g)
The shunt resistance R can be calculated from the fact that as the meter - movement and the shunt
s
are connected in parallel with each other, the potential difference across the meter - movement is
equal to the potential difference across the shunt.

∴ g g ( IR = − g )I I R s

I
or R = g R g ..... (14.18)
s
(
g )II −
The resistance of the shunt is usually so small that a piece of

copper wire serves the purpose. The resistance of the ammeter
is the combined resistance of the galvanometer’s meter -

movement and the shunt. Usually it is very small. An ammeter
Fig, 14.19 An ammeter is a galvanometer must have a very low resistance so that it does not disturb the
which is shunted by a proper low
resistance. circuit in which it is connected in series in order to measure the
current.

Voltmeter

A voltmeter is an electrical device which measures the potential difference in volts between two

points. This, too, is made by modifying a galvanometer. Since a voltmeter is always connected in
parallel, it must have a very high resistance so that it will not short the circuit across which the

voltage is to be measured. This is achieved by connecting a very high resistance R placed in series
h
with the meter - movement (Fig.14.20). Suppose we have a meter - movement whoso resistance is

R and which deflects full scale with a current / .
g
g
In order to make a voltmeter from it which has a range of V volts, the value of the high resistance R
h
should be such that full scale deflection will be obtained when it is
connected across V volt. Under this condition the current through

the meter - movement is / . Applying Ohm’s law (Fig.14.20 )we have
g

IR + R )
( V =
g g h
V Fig. 14.20 A galvanometer in series
R = − R ....... (14.19) with a high resistance acts as a
h I g
g voltmeter.
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