Ammeter

An Ammeter is a suitably shunted galvanometer. Its scale is calibrated to give the value of current in the circuit. To convert a galvanometer into an ammeter, a low resistance wire is connected in parallel with the galvanometer. The resistance of the shunt depends on the range of the ammeter.

Let G be resistance of the galvanometer and N be the number of scale divisions in the galvanometer. Let k denote figure of merit or current for one scale deflection in the galvanometer. Then current which produces full scale deflection in the galvanometer is

I_g = Nk

Let I be the maximum current to be measured by the galvanometer.

The voltage between points A and B is given by

V_AB = I_gG = (I – I_g) S

S = I_gG/(I–I_g)

As G and Sare in parallel, the effective resistance R of the ammeter is given by

R = GS/(G+S)

As the shunt resistance is small, the combined resistance of the galvanometer and the shunt is very low and hence, ammeter resistance is lower than that of the galvanometer. An ideal ammeter has almost negligible resistance. That is why when it is connected in series in a circuit, all the current passes through it without any observable drop.