Mercury Column Height Measures Atmospheric Pressure

Steps:

• Atmospheric pressure acts on the mercury reservoir, pushing mercury into the evacuated tube.
• Mercury rises until its weight balances the atmospheric force, creating equilibrium.
• The vertical height of this supported mercury column directly corresponds to pressure.
• Standard barometers measure this height in mmHg, where 760 mmHg is 1 atm.

Why C is correct:

• C represents the height from the reservoir surface to the meniscus top, defined by Torricelli's principle as equal to atmospheric pressure (P = ρgh, with ρ as mercury density, g gravity, h height).

Why the others are wrong:

• A: Measures vacuum space above mercury, unrelated to pressure balance.
• B: Indicates total tube length, fixed and not varying with pressure.
• D: Shows reservoir depth, constant and not indicative of atmospheric force.

Final answer: C