Question

In a potentiometer, the null point is obtained on \(7^{th}\) wire. What should we do to change the null point on \(9^{th}\) wire by keeping all other quantities same ?

• A. Decrease resistance in series with battery
• B. Increase resistance in main Circuit
• C. Decrease resistance in main circuit
• D. Increase applied e.m.f.

Hint:

A longer balance length means the potential gradient along the wire must be reduced.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
In a potentiometer, the balance length is \(l=\frac{E}{\phi}\), where \(\phi\) is the potential gradient along the wire.

Step 2: What Moving to the 9th Wire Means
The null point moves to a longer length, so the potential gradient \(\phi\) must decrease.

Step 3: How to Reduce the Gradient
The gradient is \(\phi=\frac{IR_{wire}}{L}\). Reducing the main current by increasing the resistance in the main circuit lowers it.

Step 4: Compare the options
So we should increase the resistance in the main circuit, option (B).

Final Answer:
Increase the resistance in the main circuit. \[ \boxed{\text{(B) }\text{Increase resistance in main Circuit}} \]