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Measurement of Atmospheric Pressure

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-170

2025-11-11

531

While the above experiments have demonstrated the existence of atmospheric pressure, they have given no accurate idea of its amount. The principle employed in finding its amount is the same as that of balancing columns used in finding the specific gravity of liquids. If a bottle is filled with water and inverted with its mouth under the surface, as in Fig. 1, the water will remain in the bottle. The downward pressure of water in the bottle A is counterbalanced by the downward pressure of the air upon FIG. 2 the surface of the water in B, since the downward pressure upon the surface is transmitted into an upward pressure at the mouth of the bottle.

In order to balance the entire pressure of the atmosphere by a water column, the bottle in Fig. 1 would need to be extended into a tube about 34 ft. long; but by using a liquid heavier than water, a correspondingly shorter tube can be used.

Demonstration. - Close one end of a glass tube 80 cm. long and about 6 mm. in internal diameter. Fill it nearly full with clean mercury. Close the open end with the finger, and invert it several times to remove all air bubbles clinging to the sides of the tube. Now fill the tube full, put a finger over the open end, invert it, and place the open end beneath the surface of mercury in a dish. Remove the finger carefully so that no air shall get into the tube. The mercury will fall a little, and the height at which it stands will measure the atmospheric pressure (Fig. 2).

FIG.2.