60. The Air Pump.—The air pump is used to remove air or other gases from a closed vessel. It was invented about 1650 by Otto Von Guericke, burgomaster of Magdeburg, Germany. One form of air pump is shown in Fig. 39. C is a cylinder within which slides a tightly fitting piston. R is the vessel from which the air is to be exhausted. r and u are valves opening upward. The action of the pump is as follows:
On pushing the piston down, the air in C is compressed. This opens valve r allowing the confined air to escape above the piston. The piston is then raised making the space in C a partial vacuum. The pressure in R now being greater than in C, u is pushed up and the air from R rushes into C, until the pressure is equalized. On pushing down the piston again, valve u closes and the process is repeated until the pressure in R is no longer able to raise the valve u. Some air pumps are so constructed that the valves are opened and closed automatically by the movement of the piston. With these pumps a higher degree of rarefaction can be obtained.
Air is often partially exhausted from receivers or vessels by the use of a filter pump or aspirator. A stream of water flowing through a constriction causes a reduced pressure, draws in air and carries it away, and thus produces a partial vacuum. See Fig. 40 for a section of the device.
Fig. 40.—An aspirator.
61. The Condensing Pump.—This is like the exhaust pump except that its valves are reversed. It is used in compressing illuminating gases into cylinders for use in lighting vehicles, stereopticons, Pintsch lights, gas light buoys, etc., and also for compressing air to operate air brakes, pneumatic hammers and drills, and for other uses.
The common condensing pump is the kind used for inflating tires. (See Fig. 41.) In this, a loosely fitting metal piston is attached to a disc of leather somewhat larger than the cylinder. This device is called a cup valve. On raising the piston, air rushes in from the top past the valve, but on pushing the piston down, the valve is pressed tightly against the sides of the cylinder and prevents the escape of any air. The compressed air pushes open a valve on the tire and enters it. This valve closes as soon as the pressure is lessened from outside. It is well to notice in all of these pumps that two valves are used. One holds the air already secured while the other opens for a new supply. Both valves are never open at the same time.
Fig. 41.—Condensing pump used in inflating tires.
62. Water Pumps.—The Common Lift Pump. This, the simplest pump for raising water, consists of a cylinder C (Fig. 42) connected by a pipe R to a supply of water as a cistern or well. A valve opening upward is placed at the bottom of the cylinder over the entrance to the pipe. In the cylinder is a tightly fitting piston connected by a rod to a lever for ease in action. The piston contains a valve opening upward. In operating this pump water is usually first poured into the cylinder to "prime" it. This helps to close the valves and prevents air leaking past them. When the piston is lowered the lower valve closes, the air in the cylinder being compressed pushes the upper valve open and passes above the piston. On raising the piston the upper valve closes. This forms a partial vacuum in the cylinder.