There is a subtlety here that is often missed, I feel. Yes, the
definitions of the metric units have changed several times. However, at
least in virtually every case, the sizes of the metric units have not
changed. As each new definition replaces its predecessor, drawing upon
improved realizations, the new definition is written so as to provide
the same size for the standard as the previous definition did, within
the limits of the precision that technology permits at that time.
By contrast, the inch, foot, and yard changed size in 1959 in the U.S.
For example, the yard changed size from 3600/3937 m to 0.9144 m. This
was a small change (~2 �m/m), but enough to cause surveyors to howl,
resulting in the survey foot being created to retain the old size
standard. Dozens of locally defined bushels and gallons changed size in
our country's early years when the first U.S. definitions of those
quantities were derived from averaging extant samples. Of course, since
they were defined in terms of cubic inches, I would deduce that they
changed again in 1959 changed again (~6 �L/L) due to the change in size
of the inch.
Thus for the metric system, new definitions but no change in sizes. For
American linear and volemic non-metric measures, at least one change in
size occurred due to definition changes.
Jim
[EMAIL PROTECTED] wrote:
>
> ----- Doorgestuurd bericht van "Joseph B. Fox" <[EMAIL PROTECTED]> -----
> Datum:Thu, 30 Aug 2001 14:22:45 +0100
> Van:"Joseph B. Fox" <[EMAIL PROTECTED]>
> Antwoord aan:"Joseph B. Fox" <[EMAIL PROTECTED]>
> OnderwerpRe: Questions about measurement standards
> Aan:Austin Spreadbury <[EMAIL PROTECTED]>, mail <[EMAIL PROTECTED]>,
> han maenen <[EMAIL PROTECTED]>
>
> Austin,
>
> Thank you for clarifying the exact curent definition of the metre. But
> in
> doing so, you make my point for me - as technology advances, so the way
> units are defined changes. The metre was once defined as the distance
> between two marks on a bar in S�vres. The present definition will chnge
> when a better time standard is established - which I believe is about to
> happen.
....
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, CAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
Charleston, SC 29407 phone/FAX: 843.225.6789
