Modern pumps may be divided into two classes : those which rapidly produce a fairly high vacuum, but not a vacuum of the highest degree, say down to a hundredth of a millimetre of mer cury, and those which produce a vacuum of the order of a mil lionth of a millimetre of mercury. Pumps of the former class are widely used to produce the preliminary vacuum required before the pumps of the second class can come into operation : they are therefore often called "preliminary" pumps, or "f ore pumps" (German Vorpumpe). The latter are called high-vacuum pumps.
Since rapidity of evacuation is one of the demands made on the modern pump it is necessary to arrive at some arbitrary definition of the speed of a pump which may be used to specify its performance. One suitable way of defining the speed is to say that it is the volume of gas removed per second, the volume being measured at the pressure prevailing in the vessel under going evacuation ; it is tacitly assumed that the tubes connecting the pumping system to the vessel are so wide that they do not appreciably affect the performances. Another way is to define the speed as the ratio of the decrease of pressure per second per unit volume to the pressure prevailing in the vessel. Since Boyle's law may be taken to hold approximately over the range in question in these high vacuum measurements these two definitions come to the same thing.
Even as so defined the speed of a pump differs at different pressures. It is expressed in cubic centimetres per second, the pressure at which it is measured being also specified. Thus, it is said that the speed of a certain Holweck pump is 2,500 cc. per second at .00r mm. of mercury.
Preliminary Pumps.—While any type of piston pump, such as the Geryk, can be used to produce the fore-vacuum of a few millimetres of mercury with which most high-vacuum pumps will work (for some high-vacuum pumps the degree of vacuum pro duced by an ordinary water jet pump, the so-called filter pump, suffices) the pumps in use as preliminary pumps at the present day are generally of the pattern in which the air is swept out by the help of vanes making an airtight contact between the outer casing and a revolving cylinder. As an example of such a pump
we may take Gaede's rotary box pump (Kapselpumpe), which was the first to establish itself in the modern laboratory, although the design was closely adumbrated in the seventeenth century by Prince Rupert's "waterbolt." (See E. N. da C. Andrade, Journal of Scientific Instruments, vol. v., p. 78, 1928.) Gaede's box-pump is illustrated in vertical section in fig. 5, both a section along the shaft and one at right angles to the shaft being shown. A cylinder A, mounted eccentrically in the cylindrical boring of the casing G, carries two hardened steel plates (S) with rounded ends which are kept in close contact with the walls by centrifugal action when the shaft revolves. Air is taken in at C and forced through the valve D into the chamber 0, which also serves as an oil chamber, lubricating the shaft by means of a loose ring. The chamber 0 is in free communication with the outside air by the tube J, through which the air is expelled. If C, instead of being connected to a vessel, is left free the pump acts as a very efficient blower, taking in air at C and delivering a jet at J. Many modifi cations of this pattern have been produced : the Trimount pump, for instance, has eight vanes of laminar construction, with a special device for maintaining them in contact with the walls. The pump made by the Central Scientific Company of America, called by them the Cenco Hyvac, has a single vane moving in a radial slot in the body of the pump, kept in contact with a rotating eccentric cylinder which has no vanes, the intake and outlet being close to the vane on either side. It is remarkably efficient.
A good box-pump is very rapid. The pressure in a vessel of ro litres can be reduced from atmospheric to less than r mm. of mercury in two minutes or so, and a final pressure of .oi mm. of mercury can be attained with certainty. With some of the most recent rotary box pumps, which work immersed in oil, pressures as low as .002 or even .00 i mm. of mercury have been attained, which is an astonishing performance. For rapid working two pumps are often connected in series, mounted on a common base and immersed in the same oil bath. Such an arrangement is widely used in lamp factories.