The idea of vacuum as insulation is quite old. Tracing the historical development, it goes back to 1897 when R.W. Wood first gave description of discharges in vacuum while investigating the production of X-ray tubes. The desire to produce X-ray tubes operating at high voltages, impelled the investigators to study the dielectric properties of vacuum. Ever since, the vacuum as an insulation has gradually found its application in electronic valves, microwave tubes, Klystrons, photocells, particle accelerators and separators, controlled nuclear fusion devices, etc. On the other hand vacuum insulation is applied in high voltage apparatuses such as electrostatic generators, low-loss capacitors, circuit breakers and also for outer space applications.

• At extremely low gas pressures, electron ionization process becomes inadequate to cause a breakdown because the 'mean free path' of an electron (defined as the distance an electron can travel without colliding with another particle) is very long.
• In a vacuum better than 10^-4 Torr (1.333 x 10^-2 Pa), less than 3 x 10¹² molecules per cm³ are estimated to be present and the length of the mean free path is of the order of meters. In such vacuum, an electron may cross a gap of a few cm between two electrodes without any collision.
• Therefore, unlike in gases, in vacuum the initial stage of breakdown cannot be due to the formation of electron avalanche.
• The process of multiplication of charged particles by collision in the space between the electrodes is far too insufficient to create avalanches. However, if a gas cloud forms in the vacuum, the usual kind of breakdown process can take place. Thus investigations of the breakdown mechanism in vacuum have been oriented to establish the way gas clouds could be created in a vacuum.
  Mechanism, suggests that the prebreakdown  currents that flow between vacuum insulated high voltage electrodes, frequently originate from nonmetallic emission mechanisms. These are associated with some kind of insulating/semiconducting surface oxides or impurity concentrations. From the technological point of view, the microscopic conditions of electrode surfaces continue to play an important role.
  Breakdown in vacuum is rather a complicated phenomenon involving a large number of processes with high electric field intensities. The' field emission' process, that is, the electron emission from metallic surfaces in presence of strong electric fields, established itself through a considerable amount of work performed in the 20^th century. After this a new process, a form of complex nonmetallic emission mechanism has made its break through to explain the prebreakdown conduction.