The electron microscope was only possible with the development of high vacuum technology. Mechanical pumps were available early in this century, and Gaede in Germany developed the mercury pump called a “condensation pump” during WWI and in the 1920's. In 1928, Burch in England found that a low vapor pressure oil would work in a mercury pump and the oil diffusion pump was born. They were made by DPI in Rochester, Metropolitan Vickers in England, and Leyboid in Germany. Other oils became available and in the mid-30's, vacuum evaporators were in laboratory use in England, Germany and the United States.

In the 1930's and 40's, the aim was to produce vacuum of 10-4 - 10-5 mm Hg where the mean free path was two feet or more and atoms, molecules and electrons could move this distance with little obstruction. In those days, we were all very happy to get 10-4 - 10-5 without worrying much as to what gas was left.