Systems of Electrical Units Page: 3
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in 1873 decided to base theoretical definitions in
both dynamics and electricity on the centimeter-
gram-second (CGS) system rather than the meter-
gram-second (MGS) system, mainly because in the
former the density of water is substantially unity.
It also urged the merits of the dynamical over the
gravitational units in mechanics, thus making the
gram primarily a unit of mass and not of force.
They proposed the names dyne and erg and defined
the horsepower as approximately "7.46 erg-nines 2
per second." The reporter of this committee,
Professor J. D. Everrett, published in 1875 a little
book "Illustrations of the C.G.S. System of Units."
This timely committee action gave such impetus to
the CGS system that it has since come to have
widespread application in all branches of science
and engineering. It has almost met the pious hope
of its originators that their selection should "be so
made that there will be no subsequent necessity
for amending it."
The year 1881 saw the first of a series of inter-
national electrical congresses (see sec. 10.2) which for
the next quarter of a century served as forums for
the discussion of nomenclature, units, etc., and as
authorities for the approval and promul nation of
those ideas which proved acceptable. The 1881
meeting in Paris approved of the basic status of the
CGS units, and of the parallel practical set with the
names olhm, volt, ampere, coulomb, and farad. It
also set up a Commission which in 1884 recom-
mended a legal ohm defined by a prototype mercury
column 106 cm long and 1 sq mm in cross section at
O C (i.e., approximately 0.9973 ohm).
By 1893 the 4th International Electrical Congress
at Chicago was able to crystallize the situation fur-
ther by defining the ohm, ampere, and volt in terms
of both the decimal multiples of the CGS electro-
magnetic units and also in terms of prototype stand-
ards. It passed a series of resolutions addressed to
the various governments represented, urging them
to "formally adopt [them] as legal units of electrical
measure." The prototype for the ohm was length-
ened to 106.3 cm (equivalent to about 1.000 5 ohm).
The prototype for the volt was the Clark Zn-Hg cell
to which was then assigned the value 1.434 v.
The 6th International Electrical Congress in St.
Louis in 1904 recognized the distinction between two
aspects of these developments. On the one hand,
there was an overriding necessity for prompt official
and universal conformity in the sizes of the units used
in commercial measurements. This could best be
secured by cooperative governmental actions. On
the other hand, the improvement and invention of
new and more useful nomenclatures and concepts
could best be fostered by providing a forum where
they could be discussed freely and by which the best
usages could be recognized and coordinated. Ac-
cordingly two separate resolutions were passed sug-
gesting these two parallel lines of progress. In sequel
the first led through several intermediate steps to the
inclusion in 1921 of electrical units in the scope of
the International Committee on Weights and Meas-
2 See p. 20, footnote 13.
ures (ICWM) (see glossary), while the second led to
the organization during the next few years on a per-
manent basis of the International Electrotechnical
The next major step in the first line of progress
was the International Conference on Electrical Units
and Standards at London in 1908, attended by
official delegates from 24 countries. It recognized
the basic importance of the CGS systems of units
and their decimal multiples but also recognized that
their experimental realization by absolute measure-
ment could not then be attained with the accuracies
desirable for much engineering work. It therefore
recommended as representing these and "sufficiently
near to them to be adopted for the purposes of
electrical measurements and as a basis for legisla-
tion" a separate system of "International Electrical
Units." The International Ohm defined as the re-
sistance, at 0 0C, of a column of mercury 106.300 cm
long and weighing 14.4521 g, and the International
Ampere defined as the current which would deposit
silver from an aqueous solution of silver nitrate at
a rate of 0.00111800 g/sec were basic units of this
system. In 1910 delegates from the British, Ger-
man, and French national laboratories met at the
Bureau of Standards in Washington and experimen-
tally intercompared their respective national standard
cells and resistors. From the results, values on a
unified basis were assigned to the various standards
and the units then arrived at were maintained [44, 46]
as closely as practicable, disseminated throughout
the civilized world, and used in commerce, industry,
and science until January 1, 1948.
Pursuant further to the first St. Louis resolution,
the 6th International Conference on Weights and
Measures in 1921 voted to amend the Convention
of the Meter (of 1875)3 to assume authority over
electric and photometric units.
In 1927 an Advisory Committee on Electricity
was established to advise the International Com-
mittee on electrical problems, and the facilities at the
International Bureau were enlarged to enable them
to make precise comparisons of electrical standards.
Since 1931 (except in time of war) the International
Bureau has made intercomparisons of standard cells
and standard resistors submitted periodically by the
various national laboratories. This enables each
laboratory to know how its units as maintained
compare with those of the other nations, and to make
adjustments on the rare occasions when such may
become necessary to restore uniformity.
By 1928 many experimenters felt that the situa-
tion had changed since 1908. The availability of
calibration services from national laboratories had
eliminated the need for convenient reproducibility in
protoptyes (desideratum (5)). Also, experience had
3 This multi-lateral international treaty established a self-perpetuating Inter-
national Committee on Weight and Meas ures eonsis ting of 18 scientists ap-
pointed by reason of their individual competence but with the proviso that only
one member be appointed from any one nation. This Committee supervises
the work of the International Bureau of Weights and Measures which occupies
laboratories on a plot of internationalized territory in 8Svres near Paris. The
operations of the Committee are reviewed and given formal approval by an
International Conference on Weights and Measures which normally meets
every six years and on which all nations signatory to the Convention of the Meter
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Silsbee, Francis B. Systems of Electrical Units, report, September 20, 1962; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc70445/m1/9/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.