Air-pump, a machine invented by Otto von Guericke of Magdeburg, in 1654, for the removal of the air or other gas from a closed cavity. The principle of most air-pumps is as follows: - A cylinder, with a closely fitting piston, is connected at its lower end with the receiver or enclosed volume of air, by a pipe. On working the pump-handle the piston moves downwards, and a portion of the air effects a passage through a valve in the piston. On the return of the piston this valve closes so that no more air passes through, whereas that portion which effected the passage is driven out of the cylinder through another valve at its upper end. Repetition of the motion therefore draws more and more air from the receiver.
The Sprengel air-pump, which is far more efficient, depends on a totally distinct principle. Mercury falling down a vertical tube connected laterally with the receiver is found to drag small bubbles of air with it until a very perfect vacuum is obtained. The apparatus has been used with great success in cases where almost complete exhaustion is required; as, for instance, in incandescent electric lamps.