Mercury Column Height Measures Atmospheric Pressure

Steps:

• Atmospheric pressure pushes mercury up the closed tube from the reservoir.
• A vacuum forms above the mercury column, exerting no downward pressure.
• The column height balances atmospheric pressure via P = ρgh, where ρ is mercury density, g is gravity, and h is height.
• Measure h from reservoir surface to column top for pressure value.

Why C is correct:

• C labels the height of the supported mercury column, directly equaling atmospheric pressure in mmHg per Torricelli's principle.

Why the others are wrong:

• A: Total tube length, unrelated to pressure balance.
• B: Reservoir mercury depth, varies with volume but not pressure.
• D: Distance from tube bottom to reservoir bottom, irrelevant to column support.

Final answer: C