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Conversion of galvanometer into ammeter
Aim : - To convert galvanometer into ammeters of different ranges which can be used to
measure the currents in electrical circuits.
Apparatus : - Galvanometer, ammeter, rheostat, variable power supply, different shunt
(variable) resistors and connecting terminals.
Description : - A galvanometer is a device used to detect the flow of the current, but not
to measure. Because its scale is not marked in amperes, though its deflection is
proportional to the current. Being the sensitive instrument, galvanometer cannot be used
to measure large currents , because it may cause damage to the coil of the galvanometer.
In order to avoid this damage and to use it as an ammeter, a low resistance is connected
in parallel to the galvanometer . As a result, even if a large amount of current is sent in
the main circuit, only a very small fraction of it passes through the galvanometer. The
scale is calibrated in amperes, for the total current, so as to read the current directly. To
measure the current, the ammeter must be connected in series in the circuit. The value of
shunt resistance ( RS ) depends upon the fraction of the total current required to pass
through the galvanometer.
Procedure : - Connect the circuit as shown in the figure. The variable power supply
connected gives the required potential difference to the circuit, to send the required
current through the ammeter. Also select and connect the required shunt resistance Rs,
across or parallel to the galvanometer, such that the galvanometer shows full deflection
for the required range of current. Then the galvanometer is said to be converted into an
ammeter of required range. By increasing the supply voltage, the galvanometer reading
is increased in steps of 5 divisions, starting from zero, the corresponding ammeter
readings are noted, in the table.
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Graph : - A graph is drawn, by taking galvanometer reading on X-axis and the
corresponding ammeter reading on Y- axis. It gives a straight line passing through the
origin. The graph is useful to know the current in the circuit, if the galvanometer reading
is known.
The experiment is repeated by connecting another shunt resistance ( Rs ) to get the
ammeter of a different range. So the same galvanometer can be used to get ammeters of
different ranges, just by changing the shunt resistance ( Rs ).
Precautions : - 1) The shunt resistance( Rs ) should be so selected such that for the
required range of ammeter, the galvanometer shows full deflection with in the scale.
2) The continuity
experiment.
Results : -
of connecting terminals should be checked before going to the
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Table
Ammeter range =
S.No.
mA
Galvanometer reading
( n divisions )
1.
2.
3.
4.
5.
*****
Shunt resistance added =
Ammeter reading
( mA )
Ω