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A mercury barometer works by stabilizing the level of mercury in the sealed glass tube with the atmospheric pressure on the open mercury chamber. The top of the barometer is filled with nothing (vacuum) because the mercury must have the space to expand and contract without any other force affecting it; a gas or liquid in the top would have its own pressure against the mercury and give inaccurate readings. Since the level of mercury in the barometer is dependent on the pressure of the actual atmosphere, there must be no other factor that inhibits the ability of the mercury to rise. High atmospheric pressure pushes the mercury up; low atmospheric pressure allows the mercury to drop. The barometer markings are also adjusted for temperature, as this affects the mercury's density and also its level in the barometer.
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