California State University, Stanislaus (CSUStan), teaches you how to adjust and read a mercury barometer, which is an instrument or measuring atmospheric pressure, usually for forecasting weather and determining altitude.
Step 1: Zero the barometer at the bottom.
Use the adjusting knob to carefully lower the surface of the merecury reservoir below the tip of zeroing peg. Then slowly raise the level until the tip of the peg just touches the surface of the mercury.
Step 2: Take the reading at the top.
Slide the vernier above the top of the mercury column. Then slowly lower it until the bottom edge of the vernier is parallel with the top of the mercury meniscus.
Step 3: Read the result.
Read the fixed scale to one side of the mercury to get the whole number value - in this case 766. On the sliding vernier, find the line that matches one of the lines on the fixed scale. That is the fraction - in this case 4. The final result here is 766.4 mm.
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