In the given meter bridge circuit, null point is found at 60 cm from end A. The unknown resistance S (in Ω) is

The correct answer is 90.

Step-by-Step Explanation:

A meter bridge works on the principle of a balanced Wheatstone bridge. Under the balanced condition (null point), no current flows through the galvanometer, and the ratio of the resistances in the two gaps is equal to the ratio of the corresponding lengths of the wire.

From the given circuit diagram, we can identify the following parameters:
- Resistance in the left gap = $S$
- Resistance in the right gap = 60 Ω
- Null point distance from end A, $l=60\text{cm}$
- Remaining length of the wire, $100-l=100-60=40\text{cm}$

The formula for a balanced meter bridge is given by:

$\frac{S}{60}=\frac{l}{100-l}$

Substituting the value of $l=60\text{cm}$ into the formula:

$\frac{S}{60}=\frac{60}{40}$

Now, solving for $S$:

$S=60\times \frac{6}{4}$

$S=60\times 1.5$

$S=90\text{Ω}$

Therefore, the value of the unknown resistance S is 90 Ω.