In a post office box if the position os the cell and the galvenometer are interchangd, then the:

Options

A. null point will not change
B. null point will change
C. post office box will not work
D. Nothing can be said.

Answer

To solve the question regarding the effect of interchanging the positions of the cell and the galvanometer in a post office box, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Post Office Box Setup**:
- A post office box is a device used to measure the current in a circuit. It consists of a galvanometer, a cell (battery), and resistors arranged in a specific manner.

2. **Identifying the Null Point**:
- The null point in a post office box is the point at which the galvanometer shows zero deflection. This occurs when the potential difference across the galvanometer is zero, meaning the current through it is also zero.

3. **Interchanging the Cell and Galvanometer**:
- When the positions of the cell and the galvanometer are interchanged, the basic principle of the circuit remains unchanged. The galvanometer will still measure the current flowing through the circuit.

4. **Analyzing the Effect of Interchange**:
- The null point is determined by the ratio of resistances in the circuit and the potential difference supplied by the cell. Since these factors remain constant regardless of the position of the cell and galvanometer, the null point will not change.

5. **Conclusion**:
- Therefore, after interchanging the cell and galvanometer, the null point remains the same.

### Final Answer:
- The correct statement is: **Null point will not change.**