Ask A Scientist© Environmental Earth Science Archive

name         Crystal S.
status       student
age          13

Question -   Is the early 1700 French calculation of the distance from
the equator to the North Pole correct by today's technology? They
calculated that their meter would represent one ten millionth of the
distance from the equator to the North Pole.
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The historical definition of the meter defined by the French Academy of
Sciences as
1/10,000,000 of the quadrant of the earth's circumference running from the
North Pole, to the equator, through the city of Paris (of course) was
replaced in 1889 by the distance between two scratch marks on a bar composed
of 90% platinum and 10% iridium, kept in Paris (I think, but I am not
certain). That definition, even though obviously inconvenient since it was
located in one place and the limited precision with which two scratch marks
could be located due to temperature changes etc. lasted until 1960. Then,
the meter was defined in terms of an emission line of the M=86 isotope of
the rare gas Kr -- specifically, 1 meter = 1,650,763.73 wavelengths of that
orange emission line. This of course had the tremendous advantage that any
well equipped lab had access to the primary standard of length.

The explosive advances in technology made that definition obsolete by the
1980's! Specifically, the invention of the laser made it possible to measure
the speed of light with an accuracy that surpassed the definition of the
meter itself. The international society that concerns itself with such
issues realized that the definition of the meter was caught on the horns of
a dilemma, since it was clear that the speed of light was only going to be
measured with increasing precision and accuracy. The dilemma was resolved by
assuming that the speed of light in a vacuum was constant.
There are some speculations running around now that this might not be so,
but they are speculations, and remain to be proven true. The "best"
measurement of the speed of light was 299,792,458 meters / sec. It was
decided that the definition of the the unit of length, the meter, be defined
in terms of the (constant) speed of light. So the definition of the meter is
allowed to "float" and is defined by the distance light travels in a vacuum
in 1/299,792,458 'ths of a second. This has the further advantage that so
far as we know all electromagnetic radiation, in a vacuum has the same
speed, so the definition of the meter does not depend upon a particular
substance, Kr.

This however doesn't get one completely off the hook, because the question
naturally arises, "Well! What's a second?". Currently, the second is defined
in terms of a specific transition in the electronic spectrum of the M=133
isotope of the element Cs.
Specifically, the second is the duration of 9,192,631,770 periods of the
radiation corresponding to the transition between the two hyperfine levels
of that isotope of Cs.
The advantage of this is that the second is based upon "counting" the
oscillations, which, in principle, is exact.

Look up the NIST website: http://physics.nist.gov for the very interesting
story of how fundamental constants are measured.

Vince Calder
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