 Although the Latin word vacuum means "empty", the object
of vacuum techniques is far from being spaces without matter. At the
lowest pressures which can be obtained by modern pumping methods there are
still hundreds of molecules in each cm3 of evacuated space.
According to the definition of the American Vacuum Society (1958) the term
"vacuum" refers to a given space filled with gas at pressures below
atmospheric, i.e. having a density of molecules less than about 2.5 x 1019
molecules/cm3.
The general term "vacuum" Includes nowadays about 19 orders of magnitude
of pressures (or densities) below that corresponding to the standard
atmosphere. The lower limit of the range is continuously decreasing, as
the vacuum technology improves its pumping and measuring techniques. |
