Problem : We can only increase the range of Ammeter but can't decrease why?
Doubt by Chinmaya
Solution : To Increase the range of the ammeter we need to reduce the resistance of ammeter which can be easily done by connecting a shunt resistance of small value but in order to decrease the range we need to increase the resistance of the shunt and as a result the net resistance of the ammeter will increases. But we know the resistance of an ideal ammeter must be zero and that of practical ammeter must be low. So by doing so the sensitivity of the ammeter will be reduced and the ammeter will not work as per the expectation.
Alternate answer :
We know, the value of shunt resistance required, in order to convert the galvanometer into ammeter having range from 0-I, is given by
S = IgG/(I-Ig)
If we would like to increase the range of ammeter i.e. the value of I then we can do it by decreasing the value of shunt resistance.
But if we would like to decrease the range of ammeter then I<Ig so the denominator will have a negative value and hence the shunt resistance will have a negative resistance which has no significance.
This is why we can only increase the range of ammeter but can't decrease the range of ammeter.
