On Sun, 28 Sep 2003 18:04:26 GMT, Richard Clark
wrote:

Pepys could have written 1£
that is shorter, but he did not as it was obviously not what was
tendered to the debauched man. Even the debauched man would
understand the significance of weight v. mass and how equivalencies of
1pound = umpty-ump grams does not render the term pound as mass,

Let me explain to you the difference between your mere equivalence and
a definition.

At the same time the pound was redefined around the world as
0.45359237 kg, the yard was redefined as 0.9144 m. Since then, no
specific action has ever been taken to redefine the yard, yet its
ultimate definition has changed not just once but twice.

When the yard was redefined as 0.9144 m in 1959, exactly 2 parts per
million less than the old U.S. definition, the meter was defined by
the distance between two lines on a certain platinum-iridium bar with
a Tresca (crooked x) cross-section, kept by the BIPM at the same
location where the official kilogram is kept.

The yard was then 0.9144 times the distance between those two marks.

Then in 1960 the meter was redefined as 1650763.73 wavelengths of the
orange-red emission line emitted by a certain transition in krypton-86
atoms. Thus, at that time the yard was ultimately defined as
1509458.354712 wavelengths of the same light.

Then in 1983 the definition of the meter was changed, making it so the
speed of light in a vacuum is exactly 299792458 meters per second. No
action was taken to change the definition of a yard, but it changed
nonetheless. A yard is now ultimately defined, for the present time
and until and if the meter definition ever changes again, as the
distance that light travels in a vacuum in 1143/374740572500 second.

On the other hand, suppose that one or all of the countries involved
choose to abrogate the 1959 redefinition of the yard in terms of the
meter, and instead restore some independent standard. What effect
would that have on the ultimate definition of the meter? None
whatsoever.

Likeways, if the kilogram were to be redefined as x buckyballs of
carbon-12, then the pound would automatically become 0.45359237x
buckyballs of carbon-12. OTOH, if someone restored the definition of
the pound to some independently maintained chunk of metal, that would
have no effect whatsoever on the definition of the kilogram, and the
realtionship between the pound and the kilogram would then become a
measured quantity rather than an exact definition.



Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/