The pressure given by a mercury barometer is always less than actual pressure. Why?

Explanation

To understand why the pressure given by a mercury barometer is always less than the actual pressure, we can break down the explanation into several steps:

### Step 1: Understanding the Mercury Barometer
A mercury barometer is a device used to measure atmospheric pressure. It consists of a glass tube filled with mercury, which is inverted into a basin of mercury. The height of the mercury column in the tube indicates the atmospheric pressure.

### Step 2: The Role of Surface Tension
When mercury is in contact with the glass of the barometer, it experiences a phenomenon known as surface tension. Surface tension is the cohesive force that causes the surface of a liquid to behave like a stretched elastic membrane. In the case of mercury, this results in a curvature at the surface where it meets the glass.

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