Question

In an experiment to measure the internal resistance of a cell by potentiometer, it is found that the balance point is at a length of $2\, m$ when the cell is shunted by a $5\, \Omega$ resistance, and is at a length of $3 \,m$ when the cell is shunted by a $10\, \Omega$ resistance. The internal resistance of the cell is

• A. $1.5\, \Omega$
• B. $10\, \Omega$
• C. $15\, \Omega$
• D. $1 \,\Omega$

Answer 1

Answer: (B)

$r=\left(\frac{l_{1}-l_{2}}{l_{2}}\right) \times R'=\left(\frac{l_{1}-2}{2}\right) \times 5$ ......(i)
and $r=\left(\frac{l_{1}-3}{3}\right) \times 10$ ........(ii)
On solving (i) and (ii), $r=10 \,\Omega$