Standards of length and time

Publisher Website
Google Scholar
Abstract
The Pt-Ir bar, international prototype of the metre, was one hundred times less precise as a length standard than the orange line of 86Kr by which the metre has been defined since 1960. Molecular absorption lines which happen to coincide with gas laser radiations are being studied, two of which now have their wavelengths agreed internationally to serve as standards one hundred times more precise than the Kr radiation. The present time standard is the period of oscillation of the radiation corresponding to a hyperfine transition in 133Cs on which the definition of the second has been based since 1967, the preceding definitions based on the year (adopted in 1956) and on the mean solar day (before 1956) being 104 and 106 times less precise. Ways to further increase the precision and accuracy of these and other types of frequency standards as well as values for the speed of light, are discussed.