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A galvanometer with its coil resistance 25 Ω requires a current of 1 mA for its full deflection. In order to construct an ammeter to read up to a current of 2 A, the approximate value of the shunt resistance should be
Asked in JEE Main Online 2018 · Conversion to ammeter and voltmeter
- (1) 1.25 × 10⁻³ Ω
- (2) 1.25 × 10⁻² Ω
- (3) 2.5 × 10⁻³ Ω
- (4) 2.5 × 10⁻² Ω
Answer: (2) 1.25 × 10⁻² Ω
Step-by-step solution
S = (I_g G)/(I - I_g) = (10⁻³ × 25)/(2 - 0.001).
S ≈ 1.25 × 10⁻² Ω.
Why the other options are wrong
- (1) Power-of-ten slip in converting mA to A.
- (3) Uses the full range as 1 A and also slips a power of ten.
- (4) Divides by 1 A instead of the shunt current of about 2 A.
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