Final answer:
A mercury barometer shows pressure by balancing the weight of a mercury column against atmospheric pressure, with a 760 mm height indicating 1 atm at sea level.
Step-by-step explanation:
A simple mercury barometer measures atmospheric pressure by comparing the weight of mercury in a tube to the air pressure outside. Atmospheric pressure pushes mercury up the tube, counteracted by gravity pulling it down, until equilibrium is reached. The mercury column's height is then a measure of atmospheric pressure; with a higher column indicating higher pressure. Because mercury is denser than water, the height for mercury to balance atmospheric pressure is more practical for measurement.
Standard atmospheric pressure at sea level supports a 760 mm column of mercury, correlating to 101,325 Pa or 1 atm. This column height is equivalent due to mercury's high density, which affects the hydrostatic pressure, the pressure a fluid exerts due to gravity. Thus, a mercury barometer is a convenient device for measuring atmospheric pressure precisely.