From: Douglas Jensen

Sorry - my Metrology brain kicked in - and since we are having so much fun,
I decided to chime in with the definitive answer on two definitions: The
Meter and The Second.  I don't claim to know all this stuff, but I do know
where to find it:

The origins of the meter go back to the 18th century. At that time, there
were two competing approaches to the definition of a standard unit of
length. The astronomer Christian Huygens suggested defining the meter as
the length of a pendulum having a period of one second; others suggested
defining the meter as one ten-millionth of the length of the earth's
meridian along a quadrant (one fourth the circumference of the earth). In
1791, soon after the French Revolution, the French Academy of Sciences
chose the meridian definition over the pendulum definition because the
force of gravity varies slightly over the surface of the earth, affecting
the period of the pendulum. Thus, the meter was intended to equal 10-7 or
one ten-millionth of the length of the meridian through Paris from pole to
the equator. However, the first prototype as short by 0.2 millimeters
because researchers miscalculated the flattening of the earth due to its
rotation. Still this length became the standard. (The engraving at the
right shows the casting of the platinum-iridium alloy called the "1874
Alloy.") In 1889, a new international prototype was made of an alloy of
platinum with 10 percent iridium, to within 0.0001, that was to be measured
at the melting point of ice. In 1927, the meter was more precisely defined
as the distance, at 0░, between the axes of the two central lines marked on
the bar of platinum-iridium kept at the BIPM, and declared Prototype of the
meter by the 1st CGPM, this bar being subject to standard atmospheric
pressure and supported on two cylinders of at least one centimeter
diameter, symmetrically placed in the same horizontal plane at a distance
of 571 mm from each other.

The 1889 definition of the meter, based upon the artifact international
prototype of platinum-iridium, was replaced by the CGPM in 1960 using a
definition based upon a wavelength of krypton-86 radiation. This definition
was adopted in order to reduce the uncertainty with which the meter may be
realized. In turn, to further reduce the uncertainty, in 1983 the CGPM
replaced this latter definition by the following definition:
The meter is the length of the path travelled by light in vacuum during a
time interval of 1/299 792 458 of a second.  (Writers comments:  Actually
the number I remember was 299792457.4 m/s, but I seem to recall that being
updated when something called into account the conditions under which that
was measured.  Now however, we have the speed of light defining the meter
as in):

Note that the effect of this definition is to fix the speed of light in
vacuum at exactly 299 792 458 mĚs-1. The original international prototype
of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept
at the BIPM under the conditions specified in 1889.

The current definition of the second is duration of 9 192 631 770 periods
of the radiation corresponding to the transition between the two hyperfine
levels of the ground state of the cesium 133 atom.

Now if I can only find my damn hyperfine level transition detector here

Jon Elson <[EMAIL PROTECTED]> on 03/05/2001 06:08:52 PM

Please respond to [EMAIL PROTECTED]

To:   Multiple recipients of list proteledausers
cc:    (bcc: Douglas Jensen/486394/US/EKC)
Subject:  Re: [PROTEL EDA USERS]:  Give up, there is no point (Ex: Confused
      Newbie  on Footprints)

Ian Wilson wrote:

> On 10:10 AM 6/03/2001 +1100, John Haddy said:
> >By definition, 1 inch = 25.4mm exactly. This was standardised
> >some decades ago.
> Off topic a little...
> But the metre (and hence inch) are derived quantities (variables) so all
> our footprints are totally useless! Do you hear - *useless*!
> The speed of light, c, is constant.  Any improvements in the how fast
> travels does not affect the number in metres per second.  The length of
> metre is changed instead.  So there is no point in trying to design
> footprints as the next time a physicist improves the estimate of c we
> to go back and update all out libraries.

This is actually not true.  The Metre is based on an exact number of
of the Neon emission line that is characteristic of the HeNe laser,
around 637 nM, I think.  So, the length of objects is not changed by speed
of light measurements.  And, the wavelength of the Neon atom will remain
quite stable when measured properly.


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