|
|
 |
 |
| |
|
|
|
|
| |
|
 British
Association for the Advancement of Science
In 1862 the British
Association for the Advancement of Science (BAAS) appointed
the first Commission entrusted with the task of studying electric
units. This commission consisted of physicists from various countries
and with world-wide reputations, which gave it an undeniably international
and authoritative character [2]. It undertook to extend work which
had been initiated by the German scientists C.F. Gauss and W. Weber.
One of its first achievements, in 1863, was adoption of the system
based on three fundamental units: metre, gram and
second. When in 1874 the centimetre replaced the metre, the new
system was named the absolute CGS system. Its use
was universal until the introduction at the beginning of the 20th
century of the MKSA system.
After adopting the CGS system the same commission also decided,
in 1874, to adopt ohm as the unit for resistance and volt for electromotive
force (emf). These so-called practical units had come
into use because of the inconvenient size of some of the electric
units in the CGS system.
The ohm was defined as 109 electromagnetic CGS units,
close to the resistance of a column of mercury about 1 m long and
of 1 mm2 cross-section. The volt was defined as 108
electromagnetic CGS units, close to the emf of a Daniell cell, commonly
used at that time in laboratories. Furthermore, prefixes ranging
from mega to micro were introduced for expressing multiples and
sub-multiples.
After the important part played by the BAAS, the work of six International
Congresses held between 1881 and 1904 contributed greatly to the
unification of electric and magnetic units. The last Congress was
held only a short time before the birth of the IEC.
|
| |
|
Fundamental units are base units, as
opposed to derived units.
Absolute measurements are based on the
three-dimensional system of units. They are no longer
relative measurements, that is based on comparisons.
The MKSA system uses metre, kilogram,
second and ampere as base units.
Practical units are obtained by multiplying
the absolute CGS units by integral powers of 10.
|
|
|
| |
 |
| |
|
 International
Electrical Congresses
At the time of the first International Electrical Congress in Paris
in 1881, there were in many countries no fewer than 12 different
units of emf, 10 different units of electric current and 15 different
units of resistance.
The principal result of this first congress was to give official
and international endorsement to the BAAS proposal concerning the
ohm and the volt. The ohm was now defined as “the resistance
of a column of mercury of 1 mm2 cross-section and 106,300
cm long at the temperature of melting ice”. The units ampere,
coulomb and farad were also defined.
In addition to these definitions in terms of conceptual representation,
the first Paris congress gave its attention to the material representation
of these units [3].
Later congresses were held in 1891 (Frankfurt), 1892 (Edinburgh),
1893 (Chicago), 1900 (again in Paris) and 1904 (St. Louis). The
Chicago congress laid down rules for the physical representation
of the three principal units: ohm, ampere and volt. Ohm and ampere
were defined in terms of the CGS electromagnetic system.
The Congress in Paris in 1900 dealt mainly with the contentious
question of magnetic units.
|
|
| |
|
Initiation of the IEC
When the next Congress met in St. Louis, the IEC was initiated
[2]. In fact, two permanent international commissions were proposed
with different sets of tasks:
 |
- to make a study of electric units and standards; and
- to study the unification of nomenclature and of the characteristics
of electrical machines and apparatus.
Obviously, two distinct needs were specified at the time.
First, the governments saw that it had become necessary for
commercial transactions and trade to take quick, official
and common action about the very different units that were
in use. Secondly, it appeared to be necessary to provide a
forum that would consist of scientists and in which manufacturers
as well as learned societies would be represented. Its responsibility
would be to study and to establish terminology for the whole
field of scientific and technical concepts [2].
|
|
|
| |
|
The contributions of Maxwell and Heaviside
|
| |
 |
The mathematical theory of electromagnetic phenomena had
been formulated on a three-dimensional basis by J.C. Maxwell
in 1873 but, despite many qualities, his presentation was
in some respects arbitrary [3]. In particular, he developed
two systems as extensions of the CGS system into the field
of electricity, the absolute electrostatic system and the
absolute electromagnetic system. They are respectively based
on:
- choosing the permittivity in Coulomb’s law to be
dimensionless and equal to 1; and
- choosing the permeability in the law of magnetic interaction
to be dimensionless and equal to 1.
If a given physical quantity is measured in the two different
systems of units, however, it has not only different numerical
values but also different dimensions. These facts were pointed
out in 1882 and later by O. Heaviside.
|
|
 |
Heaviside’s most important
objections [3] were that, in the case of both electricity
and magnetism, the electric field strength and the corresponding
flux density must be quantities with different dimensions.
Rather than pure numbers, the permittivity and the permeability
were quantities with a dimension. This means that Heaviside’s
presentation refers in fact to four dimensions.
Heaviside also criticised the irrational way in which the
factor 4π occurs or does not occur in the mathematical
formulas, implying that it should appear only in equations
concerning spherical geometry. He proposed to redefine the
electrical and magnetic units by making these smaller by a
factor of  ,
keeping the vacuum permittivity and vacuum permeability invariant.
Of course, this procedure would have dramatic consequences
for other quantities.
At the beginning of the 1890s, the Italian scientist and
engineer Giovanni Giorgi realized the great importance of
Heaviside’s ideas.
|
|
|
