ABSTRACT
From the practical point of view, a vacuum is defined as a space in which the pressure of gas is low compared with what is normal — that is to say, atmospheric pressure. This definition correctly allows a rather broad interpretation because the vacuums encountered span a wide range of pressure values. Thus, a pressure of the order of one tenth of atmospheric is quite adequate to exploit the force due to the difference between this and atmospheric pressure, as in vacuum molding applications. By contrast, for the acceleration of charged particles to relativistic energies in modern high-energy physics experiments, much more rarefied conditions are necessary, with gas densities that are approximately 15 orders of magnitude less than that prevailing at atmospheric pressure, and therefore at very low pressure indeed. Between these limits there is a whole spectrum of other applications at various degrees of vacuum, examples of which will be given in due course.
