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

Answer: (D)

50 cm

Answer 2

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.