Barometers Realm

The word “barometer” comes from the Greek roots baros (weight) and metreo (to measure), meaning “an instrument for measuring weight”. A mercury barometer is simply a glass tube filled with mercury, placed into a small reservoir (cistern) of mercury. The air pressing on the mercury in the cistern holds the mercury up in the tube, preventing it from spilling out. A register plate at the top of the barometer shows the strength of this pressure. The well-known 760 millimeters of mercury (or 30 inches of mercury) is simply the distance from the surface of the mercury in the cistern to the meniscus of mercury in the tube.