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|  In
Giorgi’s hands, the ideas of O. Heaviside became essential
elements both for developing new logical descriptions of electromagnetic
phenomena and for improving the system of units [3].
Already in 1896 Giorgi had criticized the peculiar dimensions of
electrical quantities in the three-dimensional system. He agreed
with Heaviside that permittivity and permeability expressed the
physical properties of the medium. Disregarding their dimension
led to strange situations, such as a resistance having the dimension
of a velocity or its inverse, or a self-induction having the dimension
of a length.
In Giorgi’s opinion, dimensions should express the true nature
of a physical quantity. He saw the need to introduce – together
with the base quantities length, mass and time – a fourth
base quantity of electrical nature: “It is evident that by
assuming the current as a fundamental concept, the definition of
any other electromagnetic quantity easily follows.”
Giorgi also had the great merit of showing that the “absolute”
system of practical units could be combined with the three
mechanical units metre, kilogram and second to constitute a single
coherent four-dimensional system of units. Four units –
metre, kilogram, second and, for instance, ohm or ampere –
could be chosen as base units from which all other practical electrical
units could be derived. This proposition resulted in a harmonic
synthesis of the practical electrical units with an acceptable set
of mechanical units. |
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| In an absolute system of practical units,
the units are defined in terms of the mechanical units.
A coherent system of units means that
the definition of the units avoids “useless coefficients”.
Rationalization includes giving physical
dimensions to ε0 and µ0,
and elimination of the factor 4π where it does
not concern spherical geometry.
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Giorgi was also a firm supporter of rationalization. His
careful approach required only a minimum of changes in existing unit conventions.
He did not modify the definitions of electric charge or magnetic flux,
limiting changes to those for permittivity, permeability, electric flux
density and magnetic field strength. This led to a highly satisfactory
solution, including the rationalization aspect, and won general acceptance
for the four-dimensional description of electromagnetism.
Giorgi’s contribution relates therefore essentially to four items:
- unification of the electrostatic and electromagnetic systems;
- elimination of the need for conversion factors;
- elimination of the fractional exponents from dimensional equations;
- the conclusion that permittivity and permeability are physical quantities
with dimensions (with the units F/m and H/m).
Giorgi’s all-embracing proposals to reformulate the theory of electromagnetic
phenomena as a four-dimensional theory, to rationalize the equations and
to integrate practical and MKS units in a single four-dimensional
unit system obtained a favourable response from many scientists, including
S.P. Thompson. However, it would still take more than 30 years before
these ideas were accepted by the responsible international organizations.
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