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

APPENDIX B 535
By the 1930's metric measures had become sufficiently important in American
industry to call for a simple factor for converting inch measurements to metric measure-
ments, and in 1933 the American Standards Association approved an American standard
inch-millimeter conversion for industrial use in which the inch was defined as 25.4
millimeters.
As a result of difficulties in interchanging precision parts and products manu-
factured during World War II, legislation was again sponsored between 1945 and 1947
to define the relation as a standard, this time as 2.54 centimeters. Although passage
again failed, the obvious usefulness of the centimeter-inch ratio led to its adoption
in 1952 by the National Advisory Committee for Aeronautics.
When the Director of the National Bureau of Standards realized that legislation
was not necessary if agreement could be reached with the other national standards
laboratories to use the conversion factor, it was proposed to them. Apart from the
Coast and Geodetic Survey, which, with its amassed calculations fixed, would be excepted,
the proposal won agreement. On July 1, 1959, the directors of the national standards
laboratories of Australia, Canada, South Africa, the United Kingdom, and the United
States adopted the equivalents, 1 yard=O.9144 meter (whence 1 inch=25.4 millimeters)
and 1 avoirdupois 592 37 kilogram. Without national legislation, the
differences between the United States inch and pound and the British inch and pound, so
important to industry and trade, were reconciled and uniformity was established in the
science and technology of the English-speaking nations.
WAVELENGTH DEFINITION OF THE METER
When the National Bureau of Standards was established in 1901, the principal
units of weights and measures were the yard and pound as defined by Mendenhall
and 0.453 592 427 7 kilogram, respectively) and the gallon and bushel as
defined by Hassler (a volume of 231 cubic inches and of 2,150.42 cubic inches, re-
spectively) These definitions remained unchanged for 58 years, and the last two are
still the official values.
Failure of the metric legislation in the United States has not, however, deterred
American contributions to continuing refinement of the value of the meter upon which our
units depend. When the original basis of the metric system on a terrestrial dimension
proved untenable, and a more serviceable and secure basis was found in an internationally
accepted definition of the meter, world. metrologists continued to seek some other natural,
physical standard that would make for higher precision and more universal reproduci-
bility. Such a physical standard became available in 1892—93 when Prof. Albert A.
Michelson, on leave from the physics department of the University of Chicago for a year's
work at the International Bureau at Sèvres, showed that the standard of length could be
replaced by reference to a specific wavelength of light, one of those in the cadmium
spectrum.
The line of research stimulated by Michelson's work was not to change the magni-
tude of the meter unit but to define it as a specified number of wavelengths. More than
a quarter of a century later, in 1927, the Seventh General Conference of Weights and
Measures provisionally adopted as a supplementary standard of length the equation,
1 meter= 1 553 164.13 wavelengths, with an accuracy within 1 part in 10 million, as the
relation between the meter and Michelson's red cadmium light wave.
20
ch. I, pp. 26—27, 30.