The Barometer
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-172
2025-11-11
32
The experiment with the glass tube filled with mercury was first made by Torricelli in 1643, and the space above the mercury column in the tube is called a Torricellian vacuum.
The mercurial barometer consists of a glass tube about 34 in. long, filled with mercury, and inverted with its lower end constantly below the surface of mercury in a cistern. It is fixed in a vertical position with a scale C graduated along the top near the end of the mercury column, the zero of this scale being the surface B of the mercury in the cistern A at the bottom (Fig. 1).
In reading the barometer a vernier scale is generally used to secure accuracy. The vernier must be brought to the top of the convex surface of the mercury, and the eye must be on a horizontal line from the top of the column; this may be secured by placing a small vertical mirror behind the top of the column, and placing the eye so that its image and the top of the column coincide. Before the height is read, the surface of the mercury in the cistern must be brought to the fixed zero. This is done by turning the screw c which raises or lowers the mercury in the cistern until it just touches the point of a pin projecting downward from the frame of the instrument, which point is the zero of the scale.
If a liquid less dense than mercury is used, the column will be correspondingly longer, and changes in it, caused by changes in atmospheric pressure, will be correspondingly greater. The glycerin barometer has a height of about 27 ft., and a change of nearly 11 in. for every change of 1 in. in the mercury barometer.
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