21 Jan
1999
21 Jan
'99
5:27 a.m.
It was thus said that the Great Philip.Belben(a)pgen.com
once stated:
certain
I'd like to define it in terms of SI units, but the Kilogram is not too
easy (yet).
Really? The meter is defined (certain frequency of light from a element for so many waves yada yada). Sea level is
defined (don't know
the
SI unit, but 780 millibars of pressure). Celcius is
defined (0 is
freezing
point of pure water at sea level, 100 boiling point of
pure water at sea
level) and that's all you need to define the gram: one cubic centimeter
of
water at 4C at sea level. That also gets you volume
(liters).
Well, sort of. The definition of a metre is actually 1/299792458 of the
distance travelled by light in vacuo in one second. Wavelengths of the
orange-red line in the spectrum of Krypton-86 went out long enough ago to
qualify for this list at least!
The definition of the kilogram is _still_ afaik "the mass of the
international prototype kilogram" (or kilogramme, I suppose).
And FWIW sea level pressure is 1013 millibars. 760 millimetres of mercury.
But kilogram = mass of 1 cubic decimetre of pure water is probably accurate
enough for most purposes as long as you don't have too much deuterium or
oxygen-18.
BTW Celsius isn't an SI unit. SI unit of temperature is the [degree]
Kelvin (K, written without the degree sign), defined as 1/273.16 of the
absolute temperature of the triple point of, oh no! It's water again!
Again, specify hydrogen-1 and oxygen-16 and it's probably accurate
enough...
See. Not as easy as it sounds, but still possible.
If you allow non-SI definitions, start with:
Second = 1/86400 mean solar day (that changes only by a couple of percent
every million years - enough that dinosaurs had a 400 day year, and ancient
Babylonians observed eclipses a few tens of miles from where modern
astronomers predicted them or at the wrong time of day)
Metre = 1/40000000 circumference of Earth. Originally 1E-7 distance from N
Pole to Equator through (I think) Paris. (Longer than SI metre by 0.02%)
Then proceed as above with Celsius and volumes of water...
Philip.