Today, what do you do when your meter sticks?. The
University of Houston's College of Engineering
presents this series about the machines that make
our civilization run, and the people whose
ingenuity created them.
My wife and I like to watch
the TV series Star Gate, SG-1. It's about
visiting alien worlds through an adjustable
wormhole. In one episode, two characters guess a
distance on some far planet. One thinks it's six
hundred yards -- the other, six hundred meters. The
joke, of course, is that meters and yards are
almost the same. The difference is only nine
percent.
We're told that the English yard is arbitrary,
while the metric system is rational and
self-consistent. Well, maybe. Let's see how it
works.
The meter was defined as one ten-millionth of the
distance from either pole to the equator. The gram
was the mass of pure water that fills a cube one
hundredth of a meter on each side. English and
metric time were both defined by Earth's rotation.
The French Revolutionary government put the metric
system in place. Then it was rescinded by Napoleon.
The French finally readopted it in the
mid-nineteenth century.
To use the metric system, you have to know the
exact pole-to-equator distance, and historian Ken
Alder tells the checkered history of that
measurement. The Revolutionary government sent two
fine astronomers out to determine the distance. One
was Delambre, low-key and easy-going. The other,
Méchain, was concentrated and intense. First
they determined a precise distance between Orleans,
France, and Barcelona, Spain. Then astronomical
data revealed what fraction that was of the total
from pole-to-equator distance.
By 1799, the effort yielded a standard platinum
meter stick. But Méchain had found an
inconsistency in his data, and he was terrified.
Lavoisier had told him, "You ... are carrying out
the most important mission that any man has ever
been charged with." Mëchain took that very
seriously.
For years, Méchain kept trying to get
consistent results. He told only Delambre about the
problem. Finally, after Méchain died of
Malaria in Valencia, Delambre realized what
Méchain had not. Earth is not a perfect
sphere. The data would always be flawed. He
published the results, but sealed the fact that
Méchain had been covering up
inconsistencies. By then, the method of least
squares had been created to get information from
scattered results, and Delambre used it to get a
best estimate from Méchain's data.
We now know that it takes ten million, plus
another two thousand, of those meters to get
from pole to equator. The standard platinum meter
is short by about twice the thickness of paper.
Today we define the meter as the distance light
travels in a certain small fraction of a second.
That doesn't match any pole-to-equator distance,
for there is no such consistent distance.
Rather, it simply reflects Delambre's and
Mëchain's two-century-old flawed measurement.
And the standard meter remains as arbitrary as the
distance from my nose to my outstretched fingers.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
Alder, K., The Mis-Measure of All Things.
Invention & Technology, Fall 2002, Vol.
18, pp 38-44.
For more on units and dimensions, see Episodes
260 and 1363.
