The red line of cadmium as a standard of wave-length

Abstract

The line 6438 of the first spectrum of cadmium was chosen by Michelson as the standard against which to measure the metre, because it was the best line he had discovered both from the point of view of its simplicity of structure and its capability of giving measurable interference fringes over very long paths. It has been used for comparison with the metre in four subsequent investigations and the wave-length 6438·4696 was adopted by the international Astronomical Union as the primary wave-length standard (or indeed the definition of the International Angstrom) in 1907. The line was produced by Michelson by exciting an H-shaped discharge tube with high tension alternating current, the lamp at the same time being raised to a temperature of 320° C with the aid of a furnace. Similar sources were used in six subsequent comparisons of the metre and the red line of cadmium. The results of these comparisons are in very good agreement, the accuracy appearing to be of the order of one part in five million, as can be seen from Table I.