Barometer: Vacuum Pump
A safer way to fill a barometer is to use a vacuum pump to create a vacuum at the top of the barometer. This draws the mercury up into the tube from the dish at the bottom. Continuing to draw a vacuum does not change the level of the mercury inside the tube. Again, the pressure is 731 millimeters of mercury. Closing the valve maintains the vacuum when the hose is removed.
Standard atmospheric pressure is 760 millimeters of mercury but barometer readings vary with weather conditions and are lower at higher elevation.
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