Basic D.C. Ammeter

       The basic d.c. ammeter is nothing but a D'Arsonval galvanometer. The coil winding of a basic        movement is very small and light and hence it can carry very small current. For large currents, the major part of current is required to bypassed using a resistance called shunt. It is shown in the Fig. 1.

Fig. 1 Basic d.c. ammeter

     The shunt resistance can be calculated as :

       Let      R_m = Internal resistance of coil

          R_sh = Shunt resistance

          I_m = Full scale deflection current

          I_sh = Shunt current

          I   = Total current

       Now I =  I_sh  + I_m 

       As the two resistances R_sh and R_m are in parallel, the voltage drop across them is same.

       The m is called multiplying power of the shunt and defined as the ratio of total current to the current through the coil. It can be expressed as,

       The shunt resistance may consist of a constant temperature resistance wire within the case of the meter or it may external shunt having low resistance.

       Thus to increase the range of ammeter 'm' times, the shunt resistance required is 1/(m-1) time the basic meter resistance. This is nothing but extension of ranges of an ammeter.

Examples