Question

An ammeter is obtained by shunting 30 ohm resistance. What additional shunt must be connected across it to double the range?

Answer 1

Suppose the current flowing through the shunt is A. Then the voltage across the shunt V is 30A. Now have to double the range the total shunt resistance should be such that the voltage drop across the shunt will be same for double current i.e 2A. So the total shunt resistance should be 15 ohm. Now suppose r is the new resistance, hence
$\frac{1}{r}+\frac{1}{30}=\frac{1}{15}\Rightarrow \frac{1}{r}=\frac{1}{15}-\frac{1}{30}=\frac{1}{30}\Rightarrow r=30$
So a new 30 ohm shunt should be added.