A multirange current meter can be constructed

Question:

 A multirange current meter can be constructed by using a galvanometer circuit shown in the figure. We want a current meter that can measure 10 mA, 100 mA, and 1 A using a galvanometer of resistance 10Ω and that produces maximum deflection for a current of 1 mA. Find S1, S2, and S3 that have to be used.

Solution:

I1 is measured as = 10 mA = IGG = (I1 – IG)(S1 + S2 + S3)

I2 is measured as = 100 mA = IG(G+S1)=(I2-IG)(S2-S3)

I3 is measured as = 1 A = IG(G+S1+S2)=(I3-IG)(S3)

S1 = 1 Ω

S2 = 0.1 Ω

S3 = 0.01 Ω

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