First laboratory investigations on using magnetically suspended rotors for vacuum pressure determination were carried out by Jesse W. Beams and co-workers in the 1940s at the University of Virginia, Charlottesville.@footnote 1@ The suspension system was recognized as useful also for the investigation of a special drag effect on high-speed rotors due to retarded gravitational force interaction as proposed by James C. Keith in 1963.@footnote 2@ For experimental verification of the Keith effect,@footnote 3@ a permanent-magnet suspension was developed in the late 1960s at the University of Bonn that later on provided the technological basis for a first practical SRG. The SRG development was initiated and supported among other magnetic suspension applications, such as turbomolecular pumps, neutron and molecular beam choppers, by George Comsa at KFA Juelich since 1974. Extensive studies by G. Messer at PTB-Berlin and other leading national standard laboratories, in particular the American National Bureau of Standards, in the late 1970s have prepared the way for commercialization of the SRG. A theory on the influence of the rotor surface roughness on the SRG calibration was proposed in 1980.@footnote 4@ Significant technical improvements have been contributed by Bernd Lindenau, KFA-Juelich, and Klaus Witthauer, RWD-Aachen. On occasion of the IVC-8 exhibition in Cannes (1980), MKS Instruments was the first company to apply for licence from KFA-Juelich, followed by Leybold AG in 1981. The commercial units were manufactured on order of the licensing companies by RWD-Aachen. Extension of the SRG operating range beyond the molecular drag regime up to atmospheric pressure was introduced in the mid 1980s and first commercialized by SAES Getters in 1996. @FootnoteText@ @footnote 1@J. W. Beams, J. L. Young, J. W. Moore, J. Appl. Phys. 17, 887 (1946) @footnote 2@ J. C. Keith, Rev. Mex. Fis. 12, 1 (1963) @footnote 3@ J. K. Fremerey, Phys. Rev. Lett. 30, 753 (1973) @footnote 4@J. K. Fremerey, Proc. 4th ICSS and 3rd ECOSS, vol. II, p.869, Cannes (1980)