From introduction: During 1948, the Isotope Research and Production Division undertook to obtain small quantities of nearly pure mercury isotopes by electromagnetic isotope separation. The possible use of a mercury isotope of even-numbered mass to establish an international standard of length required an isotopic purity greater than 99 per cent. It, became apparent that this degree of separation could only be achieved by preventing random diffusion of un-ionized mercury which otherwise would contaminate the isotope collectors.