THANKS for the EXCELLENT description of the history! The only place I
really end up dealing with MKS/CGS is when I teach a week on magnetics as
part of an introduction to electronics course. There are a fair number of
units in magnetics to begin with, then you get to double it to deal with
MKS and CGS. I sure like electronics where it's just meter - gram -
second.... But then I'm still designing circuit boards in mils
(milli-inches) and constantly multiplying and dividing by 25.4 to define
new footprints...

Harold


> Xiaofan Chen wrote:
>
>> On 10/9/05, Harold Hallikainen <harold@hallikainen.com> wrote:
>>> A recent post mentioned the CGS and MKS systems. It SEEMS to me that
>>> every unit should just be based on the basic units, ie meter, not
>>> centimeter, gram, not kilogram, etc. Why do we have CGS and MKS intea=
d
>>> of MGS?
>
>> The base unit for mass is kilogram not gram.
>
> The short answer is "it historic", the longer answer follows...
>
>
> This is one of the (few) sort-of inconsistencies in the SI. What is
> commonly called the "metric system" is actually a combination of a few
> quite separate ideas or principles, that came into play at different
> times,
> with different objectives.
>
> There was first the meter (actually, the metre, but I confess I'm too
> americanized to being able to use this spelling fluently :). A
> socio-politico-scientific innovation of the French Revolution. The idea
> was
> to put an end to the multiple standards that exist, and use only one
> standard. In this sense, the USA is already "metricated", since
> eighteenhundredsomething (found sources for 1866 and 1893) -- at least =
the
> length and mass units that are being used in the USA (inch, foot, yard,
> mile, pound) are defined in terms of the meter and the kilogram. There =
is
> no independent standard for how long an inch is; it is defined (by the =
US
> government) as being 0.0254 of whatever the SI meter is defined to be. =
In
> this section, we have the origin of the meter, the gram, the second, th=
e
> liter -- the birth of a common standard for common measures.
>
> The next idea behind the "metric system" is to use the same number syst=
em
> for measures that we use in general for other numbers. All who can
> immediately say (that is, in less than half a second) how many mils are
> 19/64th of an inch say "here" :)  There's a reason we use mils and not
> binary fractions of inches in PCB design. To end this type of conversio=
n
> problem, it was agreed upon that people would use the decimal rather th=
an
> the binary system for fractions. Even though in many places that use th=
e
> metric system simple fractions like half and quarter are still common,
> pretty much everybody knows how to transform them into the decimal numb=
er
> system in under half a second :)  Other than that, measurements are
> treated
> as numbers with a unit -- and for numbers, especially fractional number=
s,
> we all use in general the decimal system, so this was adopted for
> measurements, too. The idea here is to make calculations with all measu=
res
> the same as calculations with other numbers.
>
> Part of this is also the insight that we don't need different units for
> different sizes; we can just use powers of ten to create a new unit tha=
t's
> easy to convert. So instead of a mile with 1760 yards or 63,360 inches
> (everybody knew that, right? :) we use a kilometer with 1,000 meters or
> 1,000,000 millimeters and things get /a lot/ easier.
>
> These two ideas were part of what happened with a big bang during the
> French Revolution, around 1790. This didn't start then and there, of
> course; the underlying ideas had been floating around for a while by th=
en.
>
> The next idea is the discovery that most units can be seen as "derived"
> units, from a system of base units. And here it becomes really
> interesting.
> No matter whether you calculate energy in electric, mechanic, thermal,
> whatever terms, you end up with joules (in the SI). So jules is as much
> ampere=95volt=95second as it is newton=95meter. Deeper analysis gets yo=
u to a
> point where only a very few "base units" are needed, and all others are
> defined in terms of these base units. The first steps here were the CGS
> and
> MKSA systems. The use of the centimeter in the CGS system and the use o=
f
> the kilogram in the MKSA system are arbitrary, and partly due to the
> technical possibilities of creating a standard base unit, convenience i=
n
> calculations and other factors. The thing is that at the time the
> interrelationship between the units was discovered and widely explored,
> the
> units themselves (like the gram or the kilogram) were already largely
> defined. It is also not really relevant, for this aspect, what the base
> units are -- as long as you use the same minimal set for all other unit=
s.
> The base units for mass and length could be pound and inch, of course...
> but then, BTU and watt wouldn't fit. The idea here is consistency betwe=
en
> all units.
>
> The last idea behind the SI is the notion that it would be really good =
for
> everybody if everybody used the same measurement system. So they took t=
he
> existing systems, analyzed them, enhanced them to cover areas so far no=
t
> included (for example the inclusion of the mole to bring chemical
> calculations in the SI), and created one largely consistent system,
> defining the use of base and derived units and decimal multiplication
> prefixes. But the definition of the SI didn't happen in a vacuum, so to
> speak, it happened considering all the existing systems and standards a=
t
> the time. So rather than creating a completely new standard based on
> meter,
> gram, second, etc., they decided (in 1960) to stay within the already
> widely used MKSA system and base the SI on the meter, kilogram, second,
> ampere, kelvin, mole and candela. And we end up with that sort-of
> inconsistency that the base unit for mass has a multiplicator prefix of
> kilo, and the mass unit without any multiplicator prefix, the gram, is =
not
> a base unit. But the multiplicator prefixes and the base units come fro=
m
> different domains, so to speak, and are orthogonal concepts. Considerin=
g
> this, it's not really inconsistent, just something to be aware of (in t=
hat
> the newton for example is defined as m=95kg/s=9D and not m=95g/s=9D).
>
> Gerhard
>
>
> PS: If you think that "going metric" may be complicated, read section B=
.6
> in this NIST document http://physics.nist.gov/Pubs/SP811/appenB.html. I=
t
> describes the complicated unit conversions the NIST defines to avoid
> "going
> metric". I can't help it, but I don't see how it possibly could be cost
> effective to hold on to this multitude of length units, and define and
> redefine them over the decades in ever changing fractions/multiples of =
the
> meter :)
>
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