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

ACQUIRING NATIONAL STANDARDS
Division III (Chemistry) —Dr. William A. Noyes (Johns Hopkins University)
Dr. Henry N. Stokes (Johns Hopkins University)
[Additional personnel included 1 librarian, 1 computer, 1 draftsman, 4 clerks, 2 messen-
gers, 1 storekeeper, 4 mechanicians, 2 woodworkers, 3 apprentices, 2 laborers, 1 assistant
engineer, 1 electrician, 2 firemen, 2 watchmen, 1 janitor, 1 charwoman—a total of 58 at
the Bureau.]
But first a word about the hierarchy of standards with which the
Bureau was, as it still is, concerned.45 At the apex are the prototype stand-
ards, those of length, now defined in terms of the red radiation from krypton
86, and mass, the platinum-iridium kilogram cylinder maintained by the Inter-
national Bureau of Weights and Measures at Sèvres; and of time and temper-
ature, based on the revolution of the earth around the sun and the freezing
and boiling points of water (now, the triple point of water) 46 These are the
standards which, 'with certain defining relationships, fix the size of all units in
a measuring system and are absolute in the sense that they do not depend on
any other standards.
National standards are those which fix the prototype or international
value on a national basis, as in the instance of the copies of the prototype
meter and kilogram maintained at the Bureau; or are derived standards,
such as the standards of frequency, volume, or electricity, depending by
definition upon natural or material standards of the prototype category.47 /
Thus until the establishment of the absolute ohm in 1948, the ohm was
defined by an act of Congress of 1894 as "the resistance offered to an unvary-
ing electric current by a column of mercury at the temperature of melting
ice, 14.4521 grams in mass, of a constant cross-sectional area, and of the
length of 106.3 cm." 48
The nomenclature for standards of measurement has itself never been entirely stand-
ardized. What are called prototype standards are also known as international stand-
ards. Primary (now, reference) standards were those either maintained at the PTR or
constructed as such by the Bureau and intercompared with the standards abroad.
Secondary and working (now, derived and calibration) standards were lower orders of
primary standards.
The hierarchy of standards described here is largely based on A. G. McNish, "Classifica-
tion and nomenclature for standards of measurement," IRE Trans. Instru. 1—7, 371
(1958), and "Measurement standards report," ISA J., February 1961, pp. 1—40.
46 As the standard of length, long based, as Stratton knew it, on the international meter
bar at Sèvres, gave way to the wavelength of krypton 86 light, with superior standards
possible in mercury 198 and later sources, so the standard of time, long based on the
ephemeris second, is now provisionally based on the resonance frequency of the cesium
atoms in the atomic clock. See ch. VIII, pp. 462—463, 477.
See flow chart in NBS C531 (1952), p. 2, for the Bureau's experimental establishment
of the eletrical units by absolute measurement.
For the absolute ohm, see ch. VI, p. 337.
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