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= Internal resistance of coil
          Rsh = Shunt resistance
          I= Full scale deflection current
          Ish = Shunt current
          I   = Total current
       Now I =  Ish  + I
       As the two resistances Rsh and Rare 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.