While using a post office box the keys should be swiched on in the following order ,

Options

A. first cell key the and then galvanometer key.
B. first the galvanometer key and then cell key.
C. both the keys simultaneously.
D. any key first and then the other key.

Answer

To solve the question regarding the correct order of switching on keys in a post office box, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Circuit**:
- In a post office box setup, there are typically multiple keys (switches) that control different parts of the circuit. The main components usually include a cell (battery) and a galvanometer.

2. **Identify the First Key to Switch On**:
- The first key that should be switched on is the **cell key**. This is essential because the cell provides the necessary voltage to complete the circuit.

3. **Switch On the Cell Key**:
- By switching on the cell key, the circuit is completed, allowing current to flow. This is the first action we take.

4. **Identify the Second Key to Switch On**:
- After the cell key is switched on, the next key to be activated is the **galvanometer key**. The galvanometer measures the current flowing through the circuit and shows a deflection.

5. **Switch On the Galvanometer Key**:
- Once the cell key is on, we switch on the galvanometer key. This allows us to observe the current in the circuit through the galvanometer.

6. **Conclusion**:
- Therefore, the correct order of switching on the keys is: **Cell Key (A) first, followed by Galvanometer Key (B)**.

### Final Answer:
The correct order is: **A (Cell Key) first, then B (Galvanometer Key)**.

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