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?

Answer 1

Let R be the resistance of the shunt used.
Coil resistance, g = 25 Ω
Current through the coil, i_g = 1 mA
Total current i through the circuit in which the ammeter is connected = 2 A
Since R and g are connected in parallel,
$$\begin{gathered} g\times i_{\mathrm{g}}=R\left(i-i_{\mathrm{g}}\right) \\ \Rightarrow R=\frac{g\times i_{\mathrm{g}}}{i-i_{\mathrm{g}}} \\ \Rightarrow R=\frac{25\times 1\times {10}^{-3}}{2-{10}^{-3}} \\ \Rightarrow \mathrm{R}=1.25\times {10}^{-2}\mathrm{\Omega } \end{gathered}$$