A moving coil galvanometer of resistance $100 \Omega$ shows a full scale deflection for a current of

Question

A moving coil galvanometer of resistance $100 \Omega$ shows a full scale deflection for a current of 1 mA . The value of resistance required to convert this galvanometer into an ammeter, showing full scale deflection for a current of 5 mA , is $\_\_\_\_$ $\Omega$

Professor PC Thomas Classes · Answer

To convert a galvanometer into an ammeter, a shunt resistance ($S$) is connected in parallel with it. The potential difference across both branches must be equal: $I_g R_g = (I - I_g) S$. Here, $I_g = 1 	ext{ mA}$, $R_g = 100 \Omega$, and the total current to be measured is $I = 5 	ext{ mA}$. Substituting the values: $1 	ext{ mA} 	imes 100 \Omega = (5 	ext{ mA} - 1 	ext{ mA}) 	imes S \implies 100 = 4S \implies S = 25 \Omega$.