Measures for Progress: A History of the National Bureau of Standards

TECHNOLOGICAL VS. BASiC RESEARCH 463
He announced it the ultimate in a length standard.96 Three years later the
tentative standard was made available to science and industry in the form of
the NBS-Meggers Mercury 198 Lamp.
The 13 Meggers lamps initially distributed in 1951 were capable of
calibration to a precision of 1 part in 100 million, as opposed to the 1 part
in 10 million possible with the standard meter. With further refinement—
in particular, the use of an atomic beam of Hg195 instead of the vapor, to
narrow the line and overcome the slight effect of temperature on the Meg.
gers lamp—the Bureau looked forward to extending that accuracy to one part
in a 'billion.97
Funds for a program to try the atomic beam method were not then
available and it was 1959 before the work was completed. Meanwhile, the
Bureau urged adoption of the mercury 198 lamp as the international stand-
ard, considering it a simple and excellent working standard for length
measurements. Although the Russian standards laboratories favored the
cadmium lamp, the other laboratories abroad settled on the krypton 86 lamp,
proposed. originally by the Physikalisch.Technische Bundesanstalt (the West
Meggers, in J. Opt. Soc. Am. 38, 7 (1948), and Sci. Mo. 68, 3 (1949).
The standard meter, etched on a platinum-iridium bar maintained at the International
Bureau, had been the world's standard of length since 1889 (see ch. I, p. 30). The
cadmium line, first proposed by Michelson in 1893, had never been adopted. Though
widely used, its structure limited the precision attainable. Time, however, and superior
radiations witnessed notable improvements in the measurement of light waves.
Wavelengths of light are still measured with a Michelson-type interferometer. By
splitting a beam of light, the interferometer permits its speed and wavelength to be
measured with extraordinary accuracy. Each normal element or isotope of an atomic
element, when made incandescent with high-frequency radio waves, emits a light with
a characteristic and unchanging wavelength of its own. The wavelengths are measured
in terms of the angstrom, named for A. J. Angstrom who introduced the scale of wavelengths
in 1868, his unit representing the ten-millionth part of a millimeter or
meter.
The light waves from some elements and isotopes can be measured more accurately
than others, among them the red line of cadmium vapor, the orange line of krypton
gas, the green light of mercury vapor, and, sharper than these, the green light of the
unidirectional mercury atomic beam. Under excitation, the radiation of the atoms of
these elements or isotopes is seen as measurable fringes of light between the parallel
metallic-coated plates of the interferometer.
In establishing a standard of length, a gage block, meter bar or similar known quantity
is measured directly against the wavelength seen in the interferometer, the spectroscopist
measuring the half-width of the wave under observation and from that deriving precise
values, in terms of angstroms, for the length standard. See NBS M248 (1962), pp.
9—11.
°'
NBS Annual Report 1947, p. 197; RP2091, "Lamps and wavelengths of mercury 198"
(Meggers and Westfall, 1950); NBS Annual Report 1951, pp. 6, 29.