Question

In metre bridge experiment, null point is obtained at 20 cm from left end of the wire, when resistance X is balanced against another resistance Y (X < Y). To balance a resistance 4 X against Y, the new position of the null point from the same end will be

• A. 80 cm
• B. 60 cm
• C. 40 cm
• D. 50 cm

Answer 1

The Correct Option is D

According to the meter bridge principle
\(\frac {X}{Y}\) = \(\frac {20}{80}\)
\(\frac {X}{Y}\) = \(\frac {1}{4}\)
Y = 4X
Now 
\(\frac {4X}{Y}\)= \(\frac {l}{100-l}\)
Put Y = 4X
\(\frac {4X}{4X}\) = \(\frac {l}{100-l}\)
1 = \(\frac {l}{100-l}\)
100-l = l
2l = 100
l = \(\frac {100}{2}\)
l = 50 cm
Therefore the correct option is (D) 50 cm.