Work on quantities and units
From the date of the creation of these commissions, those responsible
for the two fields of standardization acted separately. The first
commission on electric units and standards met in London in 1908.
It dealt with the units and their physical representation.
The representatives of the national institutions or governments
at this conference adopted a set of fundamental units, defined as
decimal multiples of the corresponding electromagnetic CGS units,
and another one forming a system to represent the fundamental units
that was sufficiently close to the fundamental units to serve for
purposes of measurement.
These international units were based on the “international
ohm”, defined in terms of a column of mercury, and the “international
ampere”, defined in terms of the deposition of silver by an
electric current.
The IEC also began its work on terminology in 1908,
in the first Technical Committee (TC
1) to be appointed. Its title was the “Advisory Committee
on Nomenclature”.
It was not until 1927 that TC 1 dealt with the study of various
outstanding problems concerning electrical and magnetic quantities
and units. Discussions of a theoretical nature were opened at which
eminent electrical engineers and physicists considered whether magnetic
field strength and magnetic flux density were in fact quantities
of the same nature. As disagreement continued, the IEC decided on
an effort to remedy the situation. It instructed a task force to
study the question in readiness for the next meeting.
After intensive correspondence among its members, the task force
recommended (among other items) examination of whether it would
be appropriate to select, side-by-side with the CGS system, an absolute
and rationalized system for all the practical units. This could
be the system proposed by Giovanni Giorgi in 1901 (metre, kilogram,
second, international ohm) or the Dellinger-Bennett system (centimetre,
10–7 gram, second). Either system would have the advantage
of abolishing the then existing set of electromagnetic and electrostatic
units found in the CGS system. Either system would also avoid the
need to introduce at every turn the troublesome coefficients c0,
c02 or their reciprocals, c0
being the speed of propagation of electromagnetic waves in vacuum.
In 1930 in Stockholm, and based on the recommendations of the task
force, TC 1 took the following decisions which were ratified in
the same year in Oslo [2]:
- that the question of names to be allocated to magnetic units
should not be considered until general agreement had been reached
on their definitions;
- that the formula B = µ0 H represents
the modern concept of the physical relations for magnetic conditions
in vacuum; in this expression µ0
possesses physical dimensions;
- in the case of magnetic substances, the above formula becomes
B = µH, in which µ has the same physical
dimension as µ0. It follows that the relative
permeability of a magnetic substance is a number equal to µ
/ µ0.
These decisions were reinforced by proposals for the definition
of the following magnetic quantities:
- magnetic field strength;
- magnetic flux density;
- magnetic flux;
- magnetomotive force;
- magnetic permeability.
The much discussed question of the difference between the nature
of the quantities H, magnetic field strength, and B,
magnetic flux density, was finally settled. TC 1 was now able to
turn to two other most important questions: first, extension of
the existing set of practical units into a coherent practical system
of physical units; and secondly, rationalization of the electromagnetic
field equations.
In 1931, TC 1 decided to subdivide its field of study into three
categories:
- Section A: Vocabulary
- Section B: Electrical and magnetic magnitudes and units
- Section C: Letter symbols.
In Paris in 1933, following discussion of a resolution
of the American Committee of the International
Union of Pure and Applied Physics (IUPAP), Section B of TC 1
submitted a resolution to replace the CGS system of units by a more
practical system:
 “Section
B of the Advisory Committee No.1 on Nomenclature, having heard with
great interest the communication from Mr. Giorgi on the MKS system,
and endorsing the resolution adopted by the American section of
the International Union of Pure and Applied Physics at Chicago in
June 1933, decides to invite the National Committees to give their
opinion on the extension of the series of practical units at present
employed in electrotechnics by its incorporation in a coherent system
having as fundamental units of length, mass and time, the metre,
the kilogramme and second, and as fourth unit either that of resistance
expressed as a precise multiple 109 of the CGS electromagnetic
unit or the corresponding value of the space permeability of a vacuum.”
At the meeting in Scheveningen, in 1935, TC 1 took the almost unanimous
decision, following on the proposal of its Section B, to adopt under
the name of “Giorgi System” the system with four basic
units comprising metre, kilogram and second plus a fourth unit to
be chosen later.
In view of the importance of the questions dealt with by Section
B, it was also decided in 1935 to entrust all questions concerning
electrical and magnetic magnitudes and units to a special Study
Committee to which the title “Advisory Committee on Electric
and Magnetic Magnitudes and Units” was given with the number
24. The title by itself very briefly but clearly summarized the
scope of TC 24.
In 1938, TC 24 held its first meeting in Torquay. This meeting
was chiefly concerned with the problem of either choosing the fourth
unit in the Giorgi system or finding a connecting link between the
electrical and mechanical units of the same system. It recommended
as a connecting link the permeability of free space with the value
of µ0 = 10–7 H/m in the
unrationalized system or µ0 = 4π ·
10–7 H/m in the rationalized system.
Also, the TC recognized that any one of the practical units already
in use – ohm, ampere, volt, henry, farad, coulomb and weber
– could equally serve as the fourth fundamental unit.
Unfortunately, the Second World War interrupted the work of the
IEC, including that of TC 24. But, at its first post-war meeting
held in Paris in July 1950, the committee finally settled the question
of the choice of fourth unit by recommending the ampere.
|
