A galvanometer with a full-scale deflection of 20 mA is Converted to read 8 K by connecting a 395 Ω resistor in series with it. Determine the internal resistance of the galvanometer

A. 2.5Ω
B. 5.0Ω
C. 8.0Ω
D. 10.0Ω

Correct Answer:

Option B = 5.0Ω

Explanation

r = resistance of the galvanometer

l = current through galvanometer = 20mA or 0.02A

V1 = p.d across the galvanometer = I X r = 0.02 X r

: V1 = 0.02r

V2 = p.d across the multiplier = 8 – 0.02r

R = resistance of the multiplier = 395Ω

where R = V/I

–> 395 = 8−0.02r0.02
CROSS MULTIPLY

8−0.02r = 395 * 0.02

8−0.02r = 7.9

–> 0.02r = 8 – 7.9

∴ r = 0.10.02
= 5Ω