A multi-range current meter can be constructed by using a galvanometer circuit as shown in Fig. 4.5. We want a current meter that can measure 10mA, 100mA and 1A using a galvanometer of resistance 10Ω and that produces maximum deflection for current of 1mA. Find S1, S2 and S3 that have to be used


The internal resistance of the galvanometer is 10ohms.


So the voltage drop across it is V=10-2V =10mV


Applying KVL for 10mA. [1mA through galvanometer and 9mA through s1+s2+s3]


10=9(s1+s2+s3)


(s1+s2+s3) =10/9 eq.1


In case of 100mA (1mA through galvanometer+ s1 and 9mA through s2+s3)


99(s2+s3) =10+s1


(s2+s3) =(10+s1)/99 eq.2


In case of 1A (1mA through galvanometer+ s1+ s2 and 9mA through s3)


999s3=10+s1+s2 eq.3


Putting eq 2 in Eq.1;












1