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:

From the given meter bridge circuit diagram, we can identify the following parameters:
- Resistance in the left gap: $S$
- Resistance in the right gap: $60\Omega$
- Null point (balancing length) from end A: $l_1=60\text{cm}$

Since the total length of a meter bridge wire is $100\text{cm}$, the length of the remaining wire segment from the null point to end B is:

$100-l_1=100-60=40\text{cm}$

According to the balancing condition of a Wheatstone bridge (on which the meter bridge works):

$\frac{\text{Resistance in Left Gap}}{\text{Resistance in Right Gap}}=\frac{\text{Balancing Length from A}}{\text{Remaining Length from B}}$

Substituting the values into this formula, we get:

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

Simplifying the denominator on the right-hand side:

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

Now, solving for the unknown resistance $S$:

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

$S=60\times 1.5$

$S=90\Omega$

Thus, the value of the unknown resistance $S$ is $90\Omega$.