Glass U Tube with stopcock mounted on hard wooden stand. One arm of the tube is closed and completely filled with Mercury. Vacuum connection is made to the other arm. When Pressure is too low to support column of mercury, it beings to tail in the tube. The movable scale is used to measure height of column.
Unit has scale graduated from 0 to 100 mm on both sides of the central zero. Difference between levels of two mercury columns measure vacuum applied to gauges.
Heavy wall glass Manometer tubes has constrictions to prevent mercury surge from breaking glass if vacuum is unexpectedly high.