A vacuum is a space from which the gas has been removed. In fact it is impossible to obtain a perfect vacuum as any material surrounding a vacuum will have a vapour pressure and will thus release particles into the vacuum. In general use the term refers to gases at very low pressures such as exist at the limit of the earth' s atmosphere. The nearest to a perfect vacuum is in space, where the concentration of particles may be as low as one per cubic centimetre, a level of vacuum that has been unable to be replicated on Earth.
Imperfect vacuums may be produced by an air-pump. The piston air-pump can only go on working while the air remaining in the vessel that is being exhausted has enough elasticity to lift the valves of the pump. Consequently the vacuum thus produced must always be incomplete. Mercurial air-pumps, used in exhausting the bulbs of filament lamps, etc, can be made to produce a much more nearly perfect vacuum, a pressure of only one sixty-millionth of an atmosphere being possible. Still higher degrees of exhaustion have been produced by the methods of Sir James Dewar, who employed very low temperatures. At such temperatures the air and other residual gases in an exhausted vessel or vacuum tube can be liquefied, and so a vacuum nearer perfection temporarily obtained.
By the use of dense charcoal, which, when cooled down to a temperature of some 180 to 200 degrees Celsius below zero, absorbs any gases with which it is in contact, the vacuum approaches completeness.
The measurement of a vacuum is made by observation of the character of the electric discharge in it, or by the M'Leod gauge. The latter measures the pressure of the vacuum by allowing mercury to enter. An electric discharge is ribbon-like at first, followed by a luminous glow, and a fluorescent appearance of the glass container, as the exhaustion becomes more complete. In the perfect vacuum no discharge takes place. Research Vacuum
Abbreviations
SciTech
Research Vacuum