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

HIGH POLYMERS AND BUILDING RESEARCH 477
Some of the first microwave measurements of the spectrum lines of
ammonia gas against the NBS primary frequency standard suggested that
they might serve as an invariable secondary frequency standard. A year
later, in 1949, the Bureau devised means for utilizing the vibrations of atoms
in the ammonia molecule, derived from the microwave region of ammonia
gas, to control an oscillator with which to drive a clock.
The result was the first atomic clock ever built. While its magnitude
of accuracy was only a little better than that of the 24-hour rotation of the
earth, and not as good as time based on the annual rotation of the earth
around the sun, a breakthrough had been achieved. With further refinement,
using the cesium rather than ammonia atom, and with precise control of the
radio frequency, much higher accuracy became possible. A time accuracy of
at least 1 part in 10 billion, representing an error of 1 second in 300 years,
was thus achieved, without reference to the earth's rotation or the planetary
motions.'33 Such timing is not possible with the 2,000-year-old solar time
system. And on an earth gradually slowing down, millionths of a second
become vital in projecting such feats as timing rocket launches to meet in
orbit hundreds of miles above the earth where a space platform might be
assembled.
Of special importance to astronomy, both the clock and the method of
construction represented new tools of research in technical fields where pre-
cise measurement of time and frequency are crucial, from long-range radio
navigation systems and tracking of satellites to basic research in microwave
spectroscopy and in molecular structures.134
HIGH POLYMERS AND BUILDING RESEARCH
In 1944 Dr. Robert Simha, an Austrian chemist teaching at Howard
University in Washington, came to the Bureau to give a series of lectures in
a new field of science, that of the high polymers. The word "polymers" or
"high polymers" was then less than 5 years old, and the study of the molecular
H. Lyons, "Microwave spectroscopy frequency and time standards," Elec. Eng. 68,
251 (1949); NBS Annual Report 1951, pp. 13—14; H. Lyons, "Atomic clocks," Sci. Am.
196, 71 (1957). The achievement represented an outgrowth of work started at Westing-
house by Dr. Condon. See William E. Good, "The inversion spectrum of ammonia,"
Phys. Rev. 69, 539 (1946).
With continued refinement of the cesium atomic beam apparatus, the accuracy of time
measurements increased to 1 part in 100 billion, or 200 times greater than that achieved
by astronomical means. Subsequent use of atomic hydrogen masers promised to increase
the order of accuracy still further. Meanwhile, on Oct. 8, 1964, the 12th General Con-
ference of Weights and Measures, meeting in Paris, authorized a temporary atomic defi.
nition of the second, derived from the cesium clock, as the international unit of time.
NBS TNB 48, 209 (1964).