Question

A galvanometer coil has a resistance of 15 Ω and the metre shows full scale deflection for a current of 4 mA. How will you convert the metre into an ammeter of range 0 to 6 A?

Answer 1

Resistance of the galvanometer coil, G = 15 Ω

Current for which the galvanometer shows full scale deflection, I_g = 4 mA = 4 × 10⁻³ A

Range of the ammeter is 0, which needs to be converted to 6 A.

∴ Current, I = 6 A

A shunt resistor of resistance S is to be connected in parallel with the galvanometer to convert it into an ammeter. The value of S is given as:

S = ${\displaystyle \frac{{\text{I}}_{\text{g}}\text{G}}{\text{I}-{\text{I}}_{\text{g}}}}$

= ${\displaystyle \frac{4\times {10}^{-3}\times 15}{6-4\times {10}^{-3}}}$

S = ${\displaystyle \frac{6\times {10}^{-2}}{6-0.004}}$

= ${\displaystyle \frac{0.06}{5.996}}$

≈ 0.01 Ω = 10 mΩ

Hence, a 10 mΩ shunt resistor is to be connected in parallel with the galvanometer.