A galvanometer coil has a resistance of 15 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?

Given:



Resistance of galvanometer coil, G = 15 Ω


Current for full scale deflection, Ig = 4mA


Upper limit of ammeter I = 6A


A galvanometer can detect only small currents and their direction. Thus, to measure large currents it is converted into an ammeter. A galvanometer can be converted into an ammeter by connecting a low resistance called shunt resistance in parallel to the galvanometer


Let shunt resistance = S


We know that value of shunt is given by:


…(1)


By plugging the values of Ig, I and G into Equation (1), we get,


⇒


⇒ S = 10mΩ


For converting the galvanometer into an ammeter of range 0-6 we can add a shunt of resistance 10mΩ.


31