The Air Pump
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
P-158
2025-11-10
21
is an instrument used for removing the air from any vessel with which it is connected. The possibility of doing this depends entirely upon the fact that air is elastic. A simple air pump consists of a cylinder A (Fig. 1), in which moves a piston attached to the rod B. A tube C connects this cylinder with a receiver D, from which the air is to be removed. There are two valves between the air in D and the external air: one in the base of the cylinder, and the other in the piston. These open upward, allowing the air to move in one direction only.
At E there is a stopcock so arranged that it will permit free passage for the air between the pump and the receiver, or cut the receiver off altogether. F is an air-tight glass tube which communicates with the receiver, and contains a closed manometer for measuring the degree of exhaustion.
The operation of the air pump is as follows: Suppose the piston to be at the top of its stroke. The first movement downward will slightly compress the air under it. This closes the valve at the base of cylinder A and opens the valve in the piston. The air passes through this valve, and when the piston is at the bottom of its stroke, the air in the cylinder is above it. As soon as the piston is raised, the pressure below it is decreased and the valve is closed, and as the piston rises, the air is forced out of a hole in the top of the cylinder, or through the small tube G. The cylinder below the piston is filled with air expanding from D. Each stroke is only a repetition of the first, except that the amount of air taken out diminishes with every stroke
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