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A galvanometer with full-scale deflection of 10 mA is to ...

A galvanometer with full-scale deflection of 10 mA is to be converted to a voltmeter with full-scale deflection of 5v.  if a series resistance of 498Ω is used for the conversion, the resistance of the galvanometer is ?

A.

1Ω

B.

2Ω

C.

5Ω

D.

10Ω

Correct answer is B

r = resistance of the galvanometer

l = current through galvanometer = 1mA or 0.01A

V1 = p.d across the galvanometer = I X r = 0.01 X r

: V1 = 0.01r

V2 =  p.d across the  multiplier = 5 - 0.01r


R = resistance of the multiplier = 498Ω
 

where R = \(\frac{V}{I}\) --> 498 = \(\frac{5 - 0.01r}{0.01}\)

CROSS MULTIPLY --> 498 X 0.01 = 5 - 0.01r

0.01r = 5 - 4.98

∴ r = \(\frac{0.02}{0.01}\) = 2Ω