Comparisons of wave-length standards emitted by mercury-198 lamps

The reproducibility of high-quality wave-length standards emitted from water-cooled mercury-198 discharge lamps of the electrodeless and cold-cathode (Geissler) types has been investigated by means of a Fabry-Perot interferometer. These lamps, now produced commercially in this country, contain a small quantity of the isotope $_{\ \ 8\,0}^{1\, 9\,8}$Hg with argon, as carrier gas, at a nominal pressure of 3 or 5 mm. in electrodeless and 10 mm. in cold-cathode lamps; electrodeless lamps containing only the isotope and its vapour are also available. Employing for excitation a 100 Mc./sec. oscillator with electrodeless lamps and a transformer delivering 750 V at 25 mA and 50 c./sec. with cold-cathode lamps, the wave-lengths of six $_{\ \ 8\,0}^{1\, 9\,8}$Hg lines between 4046 and 5791 angstrom, emitted from a sample of each kind of argon-filled lamp, have been measured in terms of the cadmium red line at retardations of 5 to 25 cm. These measurements, supplemented by intercomparisons of argon-filled and argon-free electrodeless lamps at a retardation of 20 cm., reveal that $_{\ \ 8\,0}^{1\, 9\,8}$Hg lines are probably subject to an asymmetrical pressure broadening effect, producing a red shift of about 0$\cdot $0001 angstrom (-0$\cdot $0004 cm.$^{-1}$) per mm. of argon. Wave-length reproducibility among different samples of the same kind of lamp with the same filling is of the order of $\pm $ 0$\cdot $000 05 angstrom, or $\pm $ 1 part in 10$^{8}$, for electrodeless lamps and $\pm $ 0$\cdot $0001 angstrom for cold-cathode lamps.