In October 1901, a very successful Italian scientist and engineer Giovanni Giorgi showed at the congress of the Associazione Elettrotecnica Italiana (A.E.I.) in Rome that a coherent system of units could be achieved by adding an electric unit to the three mechanical units (centimetre, gram, second) of the existing CGS system. The event can be considered as the birth of what is now known as the International System of Units, or SI. (Visit the IEC SI Zone for a full explanation of International System of Units.)
The history surrounding the birth of SI is an example of how the world of international standardization can truly deliver a solution that meets past, present and future market needs.
The birth of SI is inseparably linked to the personality of Professor Giovanni Giorgi. This far-sighted Italian anticipated future needs and provided as early as 1901 not only suggestions for a coherent system of units, but a full-fledged solution. His case also shows that being ahead of one’s time can draw more criticism than being behind. But fortunately, Giovanni Giorgi had the satisfaction of witnessing how, after many years of seemingly endless debate, his original proposals were accepted without major changes.
This saga is, however, not merely of historical interest. We know that specific styles of art, literature, technology, etc., tend to be superseded by later ones. Here again, Giovanni Giorgi’s legacy is exceptional. Far from being challenged by any better system, the SI (International System of Units) keeps proving its worth.
Readers may know that in Switzerland laws are not imposed by government and that even parliament does not have the power of final endorsement because this is the privilege of its citizens. In a similar way, the SI was accepted by the appropriate organizations, but a perfectly democratic vote took place and is still taking place in an informal but highly efficient way: this is the everyday use of Giovanni Giorgi’s system by the international engineering community.
All historical information presented in these pages comes from the book: "1901-2001, Celebrating the Centenary of SI - Giovanni Giorgi's Contribution and the Role of SI", published in 2001 by the IEC for the 100th anniversary of the International System of Units.
Giovanni Giorgi's contribution to the SI
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 .
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.
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.
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.
Giovanni Giorgi's life and work
Giovanni Giorgi was born in Lucca, Italy, on 27 November 1871. He graduated in engineering in Rome in 1893. His accidental death occurred on 19 August 1950 at Castiglioncello.
Giorgi’s professional career was brilliant. Some highlights include:
1897-1906 – Manager of various electrical and mechanical equipment companies
1906-1921 – Director of the technical department of the City of Rome
From 1910 – Lecturer, and later professor, in various scientific fields (University of Rome; School for Aeronautical Construction, Rome; Royal School of Engineering, Rome; Royal University of Cagliari; University of Palermo)
1935 and 1938 – Italian Delegate in IEC meetings
Prof. Giorgi’s activities and interests reflect his broad cultural and scientific background, covering numerous but not necessarily related subjects:
- Science and technology, e.g. the application of operational calculus to electromagnetism; contributions to pure and applied mathematics; analytical mechanics; relativity (including correspondence with Einstein).
- The arts, one of his many contributions to the Enciclopedia Italiana concerning the use of colours in the Middle Ages and in modern art.
- Engineering, where he was active in various technical fields such as urban and interurban electric traction, and electric power distribution systems.
- Didactic issues, where he worked on methods of disseminating scientific and technical knowledge to the non-specialized public.
- Publications, being the author of 350 scientific/technical papers and author or co-author of several text books on science and engineering, for example Verso L’Elettrotecnica Moderna, 1949, Libreria Editrice Politecnica, Milano.
Giovanni Giorgi's correspondence with Oliver Heaviside
Giorgi corresponded with British mathematician Oliver Heaviside. In Giorgi’s hands, the ideas of Heaviside became essential elements both for developing new logical descriptions of electromagnetic phenomena and for improving the system of units. View some of Giorgi's letters to Heaviside.