Page 13 - 10-phy-14 Current Electricity

Experiment Find the resistance of galvanometer by half
deflection method.
14.4
Date ________________

Background Consider a parallel combination of a galvanometer of resistance RG and a
Information resistance RS. The resistance RS is usually known as shunt resistance. This parallel
combination is connected into a circuit carrying current I as shown in Fig. 14.4.1.
As the current I enters into the parallel combination, it divides into two parts IG
and IS.

In Fig. 14.4.1

Potential drop across the galvanometer = IGRG

! CAUTIONS Potential drop across the shunt = IsRS

Ÿ Remove the insulation As the galvanometer and the shunt are connected in parallel, so
from the ends of the
connecting wires by ISRS= IGRG or IS = IG RG
means of a fine and the total current RS
sandpaper.
I = IG+ IS
Ÿ Be sure that all Substituting the value of IS
connections are tight.
I= IG çæçè 1 + RG ööø÷
Ÿ Initially the key K1 should RS
be open. Take out a high
value of the resistance R I = IG èæçç RS + RG öø÷ö
(of the order of RS
thousands of ohms) and
then close the key K1. \ IG =I RS RG
RS +
Ÿ The value of R should be
much greater than the
resistance of the
galvanometer. This is to
ensure that value of the
total current I does not
appreciably change when
shunt is connected to the
galvanometer.

RG

IG +- IG
I I
RS
A B
IS
IS

Fig. 14.4.1

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