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,

\[g \times i_g = R\left( i - i_g \right)\]

\[ \Rightarrow R = \frac{g \times i_g}{i - i_g}\]

\[ \Rightarrow R = \frac{25 \times 1 \times {10}^{- 3}}{2 - {10}^{- 3}}\]

\[ \Rightarrow R = 1 . 25 \times {10}^{- 2} \Omega\]