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Re: SI definitions



It goes roughly like this (all from memory).

1) The metric CGS was used by all physicists in mechanics.
2) Electrical engineers established some practical units,
such as A, W and V. These were very different from the
CGSE and CGSM units used by physicists.
3) In 1030s an Italian physicist, Gorgi (sp?) found a way
to bring units used by physicists close to units used
by electricians. His approach (in mechanics) was:

a) Use meters but not centimeters
b) Use kilograms but not grams.
c) Use joules instead of ergs.

4) It turns out that joule defined as above is nearly the
same as the electrical unit of energy W*s. What a coincidence!

Keep in mind that W (apm^2*ohm) was a practical unit; amp
was defined by how much metal is deposited in an electrolytic
cell while ohm was defined in terms of a mercury column (of
certain length and a certain cross section). Only minor
adjustments (on the electric side) were necessary to make the
practical electrical watt identical with the mechanical watt
(defined in terms of kg and meters). Yes, what a coincidence!
That is why the unit of mass in SI is kilogram and not gram.

5) Note that cm, the subunit of meter, was the basic unit in
CGS. Making kg the basic unit of mass in SI is not very
different from that precedent. The main architect of the
CGS system was James Maxwell.
Ludwik Kowalski

Tom Ford wrote:

Being essentially unburdened by real history, but recalling that in my
youth I had to work with TWO metric systems: cgs and MKS (where the case
was very important), I just presumed that when the SI was established, the
centimeter - gram - second system lost the toss and took dynes and ergs out
of the picture too.

Tom Ford

At 02:53 PM 11/30/01 -0600, you wrote:
A student just asked me a question I couldn't answer. (usually I can make up
something convincing). Why is the fundamental unit of mass the kilogram
rather than the gram? In other words, why the prefix on the fundamental
unit. I know it was originally defined as the mass of a liter (or (0.1 m)^3)
of water. Why didn't the French call that a gram. Am I making sense?
skip