# User:Rich

#### Introduction[edit]

I have read the proposal, Docket #:2018-05, carefully and agree that Giovanni Giorgi was essential to the development of what is now known as the International System of Units (SI). My detailed comments follow.

#### Exposition[edit]

The SI was formally created in 1960 by the 11th General Conference on Weights and Measures (CGPM). As then proposed, the SI had six base units, the ones that concern us here are the meter (m), the kilogram (kg), the second (s), and the ampere (A). Giorgi's essential contribution was to show that by adding one base unit of an electrical nature (the choice of the CGPM was the ampere), the resulting MKSA system could be made coherent. In this case, coherent means that relations between the units follow the relations of physics with no additional numerical factors needed. For instance, the quotient of electrical tension and resistance must equal a current according to Ohm's law. The units in which these quatities are measured reflect the scientific relation, so that A = V/Ω ). That is simple enough. However, constructing a coherent system that included mechanical units was problematic. For instance, electrical power in a coherent system of electrical units, the watt (W), is obviously given by W=V·A. But the unit of power in the cgs system of units that was widely used at the time is the erg, which is orders of magnitude different from the watt used by electrical engineers. Maxwell had suggested that the cgs system could be made coherent with the engineering system of electrical units if the length of the centimeter was increased by a factor of 10^{9} and the mass of the gram was decreased by a factor of 10^{-11}. Maxwell called this the QES system and he didn't really recommend it.

But Giorgi found a way. His solution has two components. The first was to find a system of mechanical units, different from the cgs but nevertheless practical (unlike the QES). By chance, the MKS system would serve his purposes. This was a boon to the International Bureau of Weights and Measures (BIPM) because the defining artefacts of the meter and kilogram had been sanctioned by the CGPM in 1889, with the BIPM having responsibility for conserving these primary standards and providing copies to countries who desired them. But Giorgi's solution required a second component. To use the MKS system for the mechanical units, the magnetic constant (magnetic permeability of vacuum) and the electric constant (the electric permittivity of vacuum) were given values whose numerical components were far from 1 and which also had units. We now take this for granted, e.g. the magnetic constant is *μ*_{0}=4π×10^{-7} henries per meter (identical to the unit newtons per ampere squared); but this was controversial at the time. (Of course the two constants are not independent since their product bears an exact relation to the speed of light, but this was well known.)

#### This reviewer's appreciation of Giorgi's most seminal contributions[edit]

So Giorgi's contribution was essential and led to the International System of Units, the system that incorporates the engineering units first used by telegraph engineers with the mechanical units of length, mass, and time. Giorgi's solution also eliminated the need for two separate electrical systems, then in use--the cgs-emu system and the cgs-esu system, since these were now combined in a single system.

As the proposers of the Milestone emphasize, Giorgi was a keen advocate of a "rationalized" system of electrical units and had an interesting epistolary exchange on this subject with Oliver Heaviside. However, this was a side issue for the SI and it was even suggested at a preliminary stage that the user could decide whether to use electromagnetic equations that are rationalized or not, because the same results could be obtained. Nevertheless, the final decision was to adopt rationalized equations for use with the SI, and this gives Maxwell's equations the look that they have today--i.e. the absence of any factor of 4π.

The proposers have done a good job of setting out the above issues and showing the historical influence of Giorgi's ideas. I strongly support this Milestone initiative.

#### Final remarks[edit]

I would now like to comment on the proposers' selection of Supporting materials. I understand that they may have chosen Arthur Kennelly's paper in the *Proceedings of the National Academy of Science* because it is freely available. This is a fine paper and it belongs in the list of Supporting materials. But Kennelly's paper written the same year and published in Transactions of the American Institute of Electronical and Electronic Engineers (merged with IRE in 1962 to become the IEEE) is far more complete and even quite brilliant in its own right, containing much historical context and including a section on "Advantages of the Giorgi System to Students of Electrical Engineering".
This paper is now available from *IEEExplore* but it is not (yet) on open access. It is my strong suggestion that this paper be made available on open access by IEEE in honor of the Milestone now under consideration, and which I strongly support. The paper in question is:
A.E. Kennelly, "I.E.C. Adopts MKS System of Units", *Transactions of the American Institute of Electrical Engineers* ( Volume: **54** , Issue: 12 , Dec. 1935 ) 1373-1384.
doi: 10.1109/T-AIEE.1935.5056934