The ammeter is nothing but a low resistance galvanometer. An ammeter is constructed by connecting a low resistance S in parallel with a pivoted type moving-coil galvanometer G. S is known as the “shunt” resistance. This makes the effective resistance of galvanometer very small. The value of S depends upon the range of the required ammeter and can be calculated as follows:

Let G be the resistance of the galvanometer of ig the maximum current that can pass through it for full scale deflection. Let I be the maximum current to be measured. Then, current ig should pass through the galvanometer and the remaining (I - i_g) through the shunt resistance S. Then the potential difference between A and B is
            VA - VB = (I - i_g)S = = i_gG
Or            S = i_g /i - i_g G.

Thus when a shunt resistance of this value is connected with the galvanometer, the galvanometer works as an ammeter of rang 0 – i amp.




For more help in Conversion of Moving Coil Galvanometer into Ammeter click the button below to submit your homework assignment.