VACUUM PUMPS. So long as the vac uum was of interest mainly to the physical laboratories the forms of pumps for produc ing vacua were delicate contrivances, largely of glass, and of very limited performance, suitable only for experimental purposes. When the vacuum became a manufacturers' necessity, the machinery for its production took on a different phase. The limit of exhaustion possible to the laboratory air-pump (q.v.) is reached when there is no longer in the system air enough to lift the valves (by its expan sion). As even the lightest valve which could be constructed ceased to work while a con siderable amount of air remained, the results were inadequate to the demands of such in dustries as, for example, making of electric lamps.
For economic purposes the vacuum pump most in use is the oil-valve pump, in which the piston is of the °bucket') form and is con tinually filled with oil, the piston valves being practically floated in the oil when the vacuum becomes of high degree. The piston works on the gaseous material between two basins of oil. With a double cylinder pump working in tandem the results are excellent, the vacuum reaching rrern of an atmosphere (14.7 pounds per square inch) in pressure. Another pump used in commercial work is a modification of the Sprengal mercury pump, the action of which is the reverse of the air-lift. (See PUMPS AND PUMPING MACHINERY). A con stantly falling column of mercury is made to float downward past a Y-branch into the mercury tube, the Y being connected with the gas reservoir to be exhausted. The mercury traps and drags the gas down with it in bubbles, discharging it into the basin below, where the gas escapes. The mercury is pumped np again to the feeding basin at the head of the column. The vacuum obtainable with this pump is of an atmosphere, but the action must be prolonged to achieve such rarefication.
For high vacuum work Gaede's molecular pump is in considerable use. It depends for its action upon the friction of gas upon the surface of a solid. Fig. 1 shows a diagrammatic downward by the swiftly moving vapor which is condensed into metallic mercury when it strikes the cool walls next the water jacket, thus forming a vacuum into which the gas flows, being urged onward by the constant stream of mercury vapor from A, A. Being
section of this pump. A cylinder (A) is made to rotate clockwise in a casing (B) in which are two openings (E and F) connected by a slot. .The opening E connects with the gaseous material to be exhausted, and the opening F is the outlet of discharge. In operation the gas is dragged iron E toward F by its friction with the revolving cylinder A, and a difference of pressure at those two points is established proportional to the speed of rotation and the internal friction of •the particular gas. At speeds of 8,000 to 12000 revolutions per minute a vacuum equivalent to of an atmosphere has been obtained. The action of this pump is very rapid.
In Fig. 2 is shown a sectional diagram of the Langmuir Condensation pump, the most rapid of all vacuum pumps. In construction it is a cylinder of metal (E, E) having within it a smaller cylinder (F), above which is placed a shield of an inverted cup-like shape. The cylinder E is surrounded by a water jacket (W). At G is an inlet for the gaseous material to be exhausted, and at 0 the out let for the gas. In operation a quantity of mercury is placed in the bottom of the cylinder, small openings being provided into the inner cylinder F so that the mercury circulates in both. Heat is supplied below F, and the mercury is vaporized, the vapor passing up to the top of F and over the edge, being deflected downward by the shield S and rushing out by the annular opening A, A. The gas is dragged crowded down to the level of 0 it flows out of the pump. The continually condensing mercury drops to the bottom of the cylinder and passes into the central cylinder, to be used over and over again. With a Langmuir pump of three inches outside diameter, the perform ance amounts to from 180 to 240 cubic inches of gas exhausted per second, the vacuum obtained being equivalent to Tram of an atmosphere.