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A galvanometer of resistance 20 ohm gives full-scale deflection when 0.2 V is applied across it. To convert it into an ammeter reading 10 A full-scale, what should be done?

1. Connect a 0.02 ohm resistor across (in parallel with) it
2. Connect a 0.01 ohm resistor across it
3. Connect a 200 ohm resistor in series with it
4. Connect a 2000 ohm resistor in series with it

Correct answer: Connect a 0.02 ohm resistor across (in parallel with) it

Solution

Ig = 0.2/20 = 0.01 A. Shunt S = Ig*Rg/(I - Ig) = (0.01*20)/(10 - 0.01) ~ 0.2/9.99 ~ 0.02 ohm in parallel.

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