[OT] History of measurement - was about laptop

[Tom Freeman]

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.