First slide

Moving coil Galvanometer

Question

In an ammeter 0.2% of main current passes through the galvanometer. If resistance of galvanometer is G, the resistance of ammeter will be

Moderate

A

$\frac{1}{499} G$
B

$\frac{499}{500} G$
C

$\frac{1}{500} G$
D

$\frac{500}{499} G$

Solution

Here, resistance of the galvanometer = G Current through the galvanometer,
$I_{G} = 0.2 % of I = \frac{0.2}{100} I = \frac{1}{500} I$
$∴ Current through the shunt,$
$I_{S} = I - I_{G} = I - \frac{1}{500} I = \frac{499}{500} I$

$A s s h u n t a n d g a l v a n o m e t e r a r e i n p a r a l l e l$
$∴ I_{G} G = I_{S} S$
$(\frac{1}{500} I) G = (\frac{499}{500}) S or S = \frac{G}{499}$
$Resistance of the ammeter R_{A} is$
$\frac{1}{R_{A}} = \frac{1}{G} + \frac{1}{S} = \frac{1}{G} + \frac{1}{\frac{G}{499}} = \frac{500}{G}$
$R_{A} = \frac{1}{500} G$

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