Question

A milliammeter of range 10 mA has a coil of resistance 1 Ω. To use it as an ammeter of range 1 A, the required shunt must have a resistance of

(a) 1/101 Ω 

(b) 1/100 Ω 

(c) 1/99 Ω 

(d) 1/9 Ω

Answer 1

Correct Answer is: (c) 1/99 Ω

i_g = 10 mA = 0.01 A 

r = 1 Ω I = 1 A

V_A - V_B = i_gr = (I - i_g)S.

S = i_gr / I - i_g = 0.01 A x 1Ω / 1 A - 0.01 A 

= 1/99 Ω.