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Instructions for Reading a Mercury Barometer

Mercury barometers are simple devices that measure air pressure. Air pressure is important in meteorology, since changes in the pressure are often helpful in predicting local weather patterns. The barometer consists of a glass tube containing a column of mercury; the space above the column in the tube is a vacuum, so atmospheric pressure is given by the formula P = pgh, where p is the density of mercury, h is the mercury column height and g is 9.8 meters per second squared. Reading a mercury barometer is a simple three-step task that doesn't require any special equipment.

Things You'll Need

Mercury barometer

Instructions

- 1
  Find the zero adjusting knob or screw at the base of the barometer below the mercury reservoir. The reservoir will have a small peg on it called the zeroing peg to show where the mercury level should be. Carefully turn the adjusting knob until the height of the mercury in the reservoir just touches the tip of the zeroing peg.
- 2
  Find the vernier at the top of the mercury column. The vernier is a piece of metal that slides up or down the barometer. Slide the vernier until the bottom is exactly parallel with the top of the mercury column inside the tube.
- 3
  Read the air pressure by comparing the marks on the vernier with the fixed scale. The fixed scale is a metal frame encircling the barometer with marks on it like the marks on a ruler. Find the mark on the fixed scale that's parallel with the height of the mercury column; this mark will give you the whole-value number for the pressure. Next, find the mark on the vernier that exactly matches one of the lines on the fixed scale. This mark will give you a decimal value that you can add to the whole-number value to find the total pressure. If the mercury column is parallel with the 70th mark on the fixed scale, for example, and the fifth line up from the bottom on the vernier matches the fixed scale, the total pressure is 770.5 millimeters of mercury.

Nature