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

STANDARDS FOR THE AGE OF ELECTRICiTY 105
(CGS). But precise values for electrical units in relation to fundamental
mechanical effects are difficult to establish, and electrical science and indus-
try needed units that could be readily reproduced in their laboratories.
By 1903 general agreement on reproducible primary electrical stand-
ards had been reached. The international ohm was arbitrarily defined as
the resistance of a specified column of mercury, the international ampere
by its rate of deposition of silver, and the international volt as a specified
fraction of the electromotive force of the Weston standard cell. Commerce
and industry, assuming that the units defined by reproducible standards were
indistinguishable from absolute units, were satisfied. But for the most precise
work, science looked to the Bureau for an accurate statement of the small
but very real difference between these reproducible units and fundamental
(absolute) units.
Defining an electrical unit was one thing, determining its value relative
to absolute units was quite another, and the standards set up in accordance
with these definitions by the national laboratories here and abroad did not
show the agreement that had been hoped for. Yet as Dorsey pointed out,
between 1903 and 1910 the silver voltameter, standard resistors, standard
cells, and instruments for comparing the standards were much improved.
The work at the Bureau on the silver voltameter and standard cell,
in terms of which current and voltage were measured, was to be of special
importance in establishing more precise values for the volt. This was true
of J. G. Coffin's construction and calculations of absolute standards of in-
ductance, completed in 1906, a consequence of Rosa and Grover's extensive
theoretical examination of inductance formulas. The work not only proved
valuable for absolute measurement but of considerable service to the electrical
industry in determining the inductance of circuit configurations.6
Earlier experience in the absolute measurement of current, the results
of which were expressed in electromagnetic units, led to Rosa and Dorsey's
painstaking experiment in 1907 which demonstrated that the measurement
of a current in electromagnetic CGS units was related to its measurement in
electrostatic CGS units by exactly the numerical value of the speed of light,
accurate to within 0.03 percent.7 An important confirmation of Maxwell's
theory of light, the investigation otherwise had little inimecliate practical
application, except as an immensely prestigious piece of work that paid
dividends in recruiting young physicists for the Bureau.
S9 and SlO (Rosa and Grover, 1903—5); S29 (Coffin, 1906).
N0TE.—S designates the Scientific Papers of the NBS, issued from 1904—28, as here-
after RP will represent a Research Paper in the Journal of Research of the NBS, 1928
to date, T designates the Technologic Papers of the NBS, 1910—28, and H the NBS series
of Handbooks. For further information on these and other Bureau publications, see
notes to app. I.
S66 (Rosa and Dorsev. 1907).