Definition of meter and second

The definition of the meter and second are somewhat revised from those
posted earlier. The following paragraphs come from William H. Hayt,
Jr. and Jack E. Kemmerly's Engineering Circuit Analyis:

In the late 1700s the meter was defined to be exactly one ten-millionth of
the distance from the earth's pole to its equator. The distance was marked
off by two fine lines on a platinum-iridium bar which had been cooled to
zero degrees Celsius. Although more accurate surveys have shown since that
the marks on the bar do not represent this fraction of the earth's meridian
exactly, the distance between the marks was nonetheless accepted
internationally as the definition of the standard meter until 1960. In
that year the General Conference based a more accurate definition of the
meter on a multiple of the wavelength of radiation of the orange line of
krypton 86. Then in 1983 the meter was defined even more accurately as the
distance that light travels through space in 1/299 792 458 second.

...

The third basic unit, the second, was defined prior to 1956 as 1/86 400 of
a mean solar day. At that time it was defined as 1/31 556 925.9747 of the
tropical year of 1900. Eight years later the second was defined more
carefully as 9 192 631 770 peridocs of the transtion frequency between the
hyperfine levels F = 4, mF = 0 and F = 3, mF = 0 of the ground state of
2S(1/2) of the atom of cesium 133, unperturbed by external fields. This
latter definition is paermanent and more reproducible than the former; it
is also comprehensible only to atomic physicists. However, any of these
definitions adequately describes the second with which we are all familiar.

I hope that answers your questions. I read your message and thought of
these few paragraphs in my Circuits book. It just took a while to dig my
circuits book from the depths of textbookland. This book has got to be one
of the most humorous textbooks around...

        Matt Banner
        bbanner_at_intgp1.att.com
Received on Fri Sep 01 1995 - 17:03:00 EDT