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

76 FOUNDING THE NATIONAL BUREAU OF STANDARDS (1901-10)
Since national standards, whether defin'itions or materials made of
precious metal or of delicate construction and necessarily preserved under
special conditions, may be impractical, or frequent use may impair their
accuracy, the Bureau maintains national reference standards, often of its own
construction, the values of which are derived directly from the national
standards, but of suitable material or form for more frequent service.
Next are the working or calibration standards, those which are ordi-
narily used in calibration and which are themselves calibrated in terms of
the corresponding reference standards. They are compared as frequently
as necessary with the reference standards and sometimes even with the
national standards.
In most instances it is the Bureau's reference or calibration stand-
ards that are the immediate source of industrial and commercial standards
and of the precision measurements of science. Against these are calibrated
the laboratory reference standards of science and industry. Whether a pre-
cision thermometer, a kilowatt hour meter, or a standard of length, weight,
or mass, it is brought to the Bureau periodically and carefully calibrated
against the Bureau's reference standard. Returned to the factory or plant,
the laboratory reference standard then becomes the basis for calibration and
adjustment of the laboratory working standards, by which shop instruments
and measuring apparatus in daily use by technicians and inspectors are
calibrated.49
This sequence of standards is of course meaningless without special
comparison equipment—longitudinal and geodetic comparators for length
standards, the balances used in comparing masses or weights, the potentiom-
eters, bridges, and consoles used in electrical measurements—by means of
which all standards of a given type are intercompared in order to determine
the order of agreement among them. Differences naturally exist between the
nominal value of any standard (except a prototype) and the value it is found
to have when compared with a known standard, by reason of differences in
their composition or construction, circumstances of measurement, or other ir-
reducible factors, but the discrepancy as an observable quantity, can be ad-
usted or compensated for, or even within certain limits may be accepted as
a permissible tolerance.
In the relatively uncomplicated world of 1904, scientifically speaking,
American industry had need for stability and accuracy of measurement
rather than high precision. Industry had little requirement as yet for work-
ing measurements closer than a thousandth of an inch, but to achieve that
with a milling machine, for example, the accuracy of the company master
standard had to be on the order of a ten-thousandth of an inch, the Bureau's
"For the operation of laboratory standards, see NBS C578, "Suggested practices for
electrical standardizing laboratories" (1956), p. 1.