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A galvanometer G of resistance 25 ohm gives full-scale deflection at 10 mA. It is converted into a multi-range ammeter using shunts R1, R2, R3 in an Ayrton (universal) shunt arrangement: range 0-10 A across terminals O and P, range 0-1 A across O and Q, range 0-0.1 A across O and R. Find R1, R2, R3 (the three shunt sections, total shunt = R1 + R2 + R3 in series, with the galvanometer across appropriate taps). Which set is correct?
- R1 = 0.025 ohm, R2 = 0.225 ohm, R3 = 2.25 ohm (Ayrton shunt sections; total = 2.5 ohm)
- R1 = 2.5 ohm, R2 = 25 ohm, R3 = 250 ohm
- R1 = 0.25 ohm, R2 = 0.25 ohm, R3 = 0.25 ohm
- R1 = 25 ohm, R2 = 2.5 ohm, R3 = 0.25 ohm
Correct answer: R1 = 0.025 ohm, R2 = 0.225 ohm, R3 = 2.25 ohm (Ayrton shunt sections; total = 2.5 ohm)
Solution
Applying the Ayrton-shunt balance for ranges 10 A, 1 A and 0.1 A with I_g = 0.01 A and total shunt 2.5 ohm gives section values 0.025 ohm, 0.225 ohm and 2.25 ohm.
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