ABSTRACT
It is customary to specify level of vacuum in terms of pressure. In technical literature on high vacuum technology the pressure is always considered to be an absolute pressure. This means that the absence of gas is taken as zero pressure without any direct reference to the atmosphere. In high-vacuum technology, pressure is not the most interesting property of the rarefied gas environment. Near the atmospheric end, the knowledge of forces on the walls of vacuum chamber is important for design. The variation of conductivity with pressure can be utilized for indirect measurement of pressure by monitoring the heat transfer rate from a heated filament to the surrounding walls kept at ambient temperatures. The terms "evaporation" and "condensation" refer to the phenomena associated with bulk substance. When the amount of liquid remaining on a solid surface is reduced to one molecular layer or less, the corresponding terms used to describe the arrival and departure of gas are "adsorption" and "desorption.".
