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Question
An ammeter is to be constructed that can read currents up to 2.0 A. If the coil has resistance of 25 Ω and takes 1 mA for full-scale deflection, what should be the resistance of the shunt used?
Solution
Let R be the resistance of the shunt used.
Coil resistance, g = 25 Ω
Current through the coil, ig = 1 mA
Total current i through the circuit in which the ammeter is connected = 2 A
Since R and g are connected in parallel,
\[g \times i_g = R\left( i - i_g \right)\]
\[ \Rightarrow R = \frac{g \times i_g}{i - i_g}\]
\[ \Rightarrow R = \frac{25 \times 1 \times {10}^{- 3}}{2 - {10}^{- 3}}\]
\[ \Rightarrow R = 1 . 25 \times {10}^{- 2} \Omega\]
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