WAY off topic. Unless you find measurements fascinating, please ignore,
and I thank you for your patience.
On 03/15/2007 09:19:55 PM, Bruce Feist wrote:
Tim wrote:
Other metric measures do have easily understandable correlations (a
litre of water weighs a kilogram, and so on).
I'm coming into this conversation late; please forgive me if I'm
saying something that's irrelevant or obvious to everyone. (I did
check older messages, but only a dozen or so.) Also, I might be
being somewhat innacurate -- my comment is based on what I remember
from learning metric in the 1970s.
The link for a meter, or rather a centimeter, is mathematical rather
than physical: a cubic centimeter is a milliliter. I don't know how
this relates to the redefinitions -- do they implicitly redefine a
liter? What's the dependency -- is a milliliter based on a
centimeter, and a gram on a milliliter, or is it reversed, or neither?
Bother. I was trying to supply an answer, and instead I've asked
more questions.
Aren't those the best answers? The ones that result in more questions??
I hit on the wikipedia for this, and got even more than I vaguely
remembered. And I'm supposed to be aware of much of this...
About 1790, there were two suggestions for defining a unit of length,
either a pendulum with a half period of one second, or one ten
millionth of one quarter of a meridian of the earth. The pendulum had
to be dropped because the earth's gravitational field varies
measurably, causing the pendulum's half perion to vary. A seven year
surveying effort (1792-1799) gave a meter off by 0.2mm or so. In 1870,
the meter was set to the distance between two marks on a
platinum/iridium bar at a temperature of melting ice. In 1960, the
standard was shifted to the orange-red emision line of Krypton-86 in a
vacuum, although the old standard is still maintained. The Krypton
definition is the one I remember, but this was replaced in 1983 by
defining the meter in terms of time and the speed of light.
The relationship between a gram and the milililer of water at 4 degree
Centigrade is apparently the original definition of the gram. (Well,
actually water at the melting point of ice, but the 4 degree point was
quickly switched to) The current standard kilogram was adopted later,
and I haven't found quite the reason.
