Question

Two resistances are connected in two gaps of a metre bridge. The balance point is 20 cm from the zero end. A resistance of $15\,\Omega$ is connected in series with the smaller of the two. The null point shift to 40 cm. The value of the smaller resistance in Ohm is

• A. 3
• B. 6
• C. 9
• D. 12

Answer 1

Answer: (C)

Let S be the large and R be the smaller resistance, from formula for meter bridge $S=\left( \frac{100-l}{l} \right)R$
$=\left( \frac{100-20}{20} \right)R=4R$
Again, $S=\left( \frac{100-l}{l} \right)(R+15)$
$=\left( \frac{100-40}{40} \right)(R+15)$ $4R=\frac{3}{2}(R+15)$ $8R-3R=45$ $5R=45$ $R=9\Omega$ .