Question

The resistance of an ammeter is $13\Omega$ and its scale is graduated for a current up to $100A$. After an additional shunt has been connected to this ammeter, it becomes possible to measure currents up to $750A$ by this meter. The value of the shunt resistance is :

• A. $20\Omega$
• B. $2\Omega$
• C. $0.2\Omega$
• D. $2k\Omega$

Answer 1

The correct option is B $2\Omega$

Let $i_a$ is the current flowing through ammeter and $i$ is the total current. So, a current $i-i_a$ will flow through shunt resistance.
Potential difference across ammeter and shunt resistance is same.
i.e., $i_a\times R=(i-i_a)\times S$
or $S=\frac{i_{a}R}{i-i_a}$ .....(i)
Given, $i_a=100A,i=750A,R=13\Omega$
Hence, $S=\frac{100\times 13}{750-100}=2\Omega$