Pressure of the Atmosphere
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-166
2025-11-11
30
Since air has weight, the layers of air near the surface of the earth are subject to a pressure due to the weight of the air above.
Demonstrations. - Tie a sheet of thin rubber over one end of a bladder glass (Fig. 1) and place it on the plate of an air pump. Remove a part of the air below the rubber by a stroke or two of the pump. The rubber will be pushed inside the glass. What supports the downward pressure of the air before the pump is worked?
Cut off the stem of a thistle tube about 4 in. from the cup. Tie a thin rubber sheet over the cup. Slip over the stem a flexible rubber tube, and draw out some of the air by suction. Pinch the rubber tube to keep the pressure constant. Hold the cup in different positions (Fig. 2), and it will be seen that the air presses equally in all directions.
ill a tumbler full of water. Slide a heavy card over the top of the tumbler, being careful that no air bubbles are left below it. Hold the card while you invert the tumbler. Remove the support from the card, and it will remain pressed against the tumbler, holding in the water. Hold the card on and turn the tumbler so that the card is vertical; when the hand is removed, the card still remains.
Select a long, clear glass bottle and fit to the neck a rubber stopper Pass through this a short glass tube with the inner end drawn down to a fine opening. Fit one end of a rubber tube to the outer end of the glass tube and connect the other end to the air pump. Exhaust the air. Pinch the tube together; hold the bottle as in Fig. 3, and pull the rubber from the glass tube when it is below the surface of the water in the dish A. The pressure of the air upon the surface of the water will force a stream through the tube, forming a fountain inside the bottle, and the water will continue to flow until the amount of water in the bottle is equal to the volume of air taken out. This is a very old experiment, called the fountain in vacuo.
FIG.2.
Procure two small bottles and put into each the end of a U-tube, fitting loosely in B (Fig. 4), and air-tight through a rubber stopper in A. Fill A half full of water; place both under the receiver of an air pump and exhaust the air. At the first stroke the water from A will begin to run into B….
Exhaust until nearly all the water is drawn over, and then let air into the receiver. The water will run back into A…
FIG.3
FIG.4
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