Why is a partially evacuated discharge tube taken for Thomson's experiment ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Discharge Tube**: A discharge tube is a sealed glass tube that contains gas at low pressure. In Thomson's experiment, it is crucial to create conditions that allow the study of cathode rays (electrons).

2. **What Does "Partially Evacuated" Mean?**: A partially evacuated discharge tube means that most of the air has been removed from the tube, but not all. This creates a low-pressure environment inside the tube.

3. **Importance of Low Pressure**: In a low-pressure environment, the density of gas molecules is significantly reduced. This is important because it allows the electrons emitted from the cathode to travel freely without being obstructed.

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