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Scientists Struggling to Make the Kilogram Right Again
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Lowell Prange
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May 28, 2003 17:04 PDT
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Anyone have a better idea for a standard for the kilogram to be based on?
| | http://www.nytimes.com/2003/05/27/science/27KILO.html
Scientists Struggling to Make the Kilogram Right Again
By OTTO POHL
BRAUNSCHWEIG, Germany — In these girth-conscious times, even weight itself
has weight issues. The kilogram is getting lighter, scientists say, sowing
potential confusion over a range of scientific endeavor.
The kilogram is defined by a platinum-iridium cylinder, cast in England in
1889. No one knows why it is shedding weight, at least in comparison with
other reference weights, but the change has spurred an international
search for a more stable definition.
"It's certainly not helpful to have a standard that keeps changing," says
Peter Becker, a scientist at the Federal Standards Laboratory here, an
institution of 1,500 scientists dedicated entirely to improving the
ability to measure things precisely.
Even the apparent change of 50 micrograms in the kilogram — less than the
weight of a grain of salt — is enough to distort careful scientific
calculations.
Dr. Becker is leading a team of international researchers seeking to
redefine the kilogram as a number of atoms of a selected element. Other
scientists, including researchers at the National Institute of Standards
and Technology in Washington, are developing a competing technology to
define the kilogram using a complex mechanism known as the watt balance.
The final recommendation will be made by the International Committee on
Weights and Measures, a body created by international treaty in 1875. The
agency guards the international reference kilogram and keeps it in a
heavily guarded safe in a château outside Paris. It is visited once a
year, under heavy security, by the only three people to have keys to the
safe. The weight change has been noted on the occasions it has been
removed for measurement.
"It's part ceremony and part obligation," Dr. Richard Davis, head of the
mass section at the research arm of the international committee.
"You'd have to amend the treaty if you didn't do it this way."
That ceremony has become a little sorrowful as the guest of honor appears
to be, on a microscopic level at least, wasting away.
The race is already well under way to determine a new standard, although
at a measured pace, since creating reliable measurements is such
painstaking work.
The kilogram is the only one of the seven base units of measurement that
still retain its 19th-century definition. Over the years, scientists have
redefined units like the meter (first based on the earth's circumference)
and the second (conceived as a fraction of a day). The meter is now the
distance light travels in one-299,792,458th of a second, and a second is
the time it takes for a cesium atom to vibrate 9,192,631,770 times. Each
can be measured with remarkable precision, and, equally important, can be
reproduced anywhere.
The kilogram was conceived to be the mass of a liter of water, but
accurately measuring a liter of water proved to be very difficult.
Instead, an English goldsmith was hired to make a platinum-iridium
cylinder that would be used to define the kilogram.
One reason the kilogram has lagged behind the other units is that there
has been no immediate practical benefit to increasing its precision.
Nonetheless, the drift in the kilogram's weight carries over to other
measurements. The volt, for example, is defined in terms of the kilogram,
so a stable kilogram definition will allow the volt to be tied more
closely to the base units of measure.
A total of 80 copies of the reference kilogram have been created and
distributed to signatories of the metric treaty. The sometimes colorful
history of these small metal cylinders underscores how long the world has
used the same definition of the kilogram.
Some of the metal plugs were issued to countries that later vanished,
including Serbia and the Dutch East Indies. The Japanese had to surrender
theirs after World War II. Germany has acquired several weights, including
the one issued to Bavaria in 1889 and the one that belonged to East Germany.
To update the kilogram, Germany is working with scientists from countries
including Australia, Italy and Japan to produce a perfectly round
one-kilogram silicon crystal. The idea is that by knowing exactly what
atoms are in the crystal, how far apart they are and the size of the ball,
the number of atoms in the ball can be calculated. That number then
becomes the definition of a kilogram.
To separate the three isotopes of silicon, Dr. Becker and his team are
turning to old nuclear weapons factories from the Soviet Union, where
centrifuges once used to produce highly enriched uranium are able to
produce the required purity of silicon.
"We need so many nines," Dr. Becker said, and Soviet uranium processors
are one of the only places to get them. "With the Russians, we're getting
about four of them," or 99.99 percent pure silicon 28.
A test crystal has already been produced, and Dr. Arnold Nicolaus, another
scientist at the German standards laboratory, is responsible for measuring
whether it is perfectly round. He has measured the crystal in a
half-million places to determine its shape.
It's probably the roundest item ever made by hand. "If the earth were this
round, Mount Everest would be four meters tall," Dr. Nicolaus said. An
intriguing characteristic of this smooth ball is that there is no way to
tell whether it is spinning or at rest. Only if a grain of dust lands on
the surface is there something for the eye to track.
Scientists from the United States, England, France and Switzerland say the
challenge of calculating the precise number of atoms in a silicon crystal
is too imprecise with today's technology so they are refining a technique
to calculate the kilogram using voltage.
"Measuring energy is easier than counting atoms," said Dr. Richard
Steiner, a scientist at the National Institute of Standards and Technology
in Washington, who is leading the international project to create the watt
scale.
In the last few weeks, he has reported that his experiments have yielded
data that are close to what they need. "Now we're into the picayune,
itsy-bitsy errors," he said, having recently corrected "totally
ridiculous" errors of 100 parts per million.
The idea of the watt balance is to measure the electromagnetic force
needed to balance a reference kilogram. As long as the gravitational field
is precisely known for the location of the experiment, the mass on the
scale can be related to power. (The gravitational field is a complicated
calculation that needs among other things constantly updated changes in
tidal forces.)
The definition of the kilogram would then be a measurement of that power
or in terms of something that could be derived from it, like the mass of
an electron. The experiment in Washington is occurring in a large
three-story structure, but in spite of the complexity and circuitous route
of calculating mass, Dr. Steiner says he is confident that his team will
have persuasive data shortly.
"In the short term, I think we'll win," he said.
Dr. Davis, who is working closely with those making the final decision
about the fate of the kilogram, says he is not so sure. "In terms of
published results, the watt balance is closer of the two," he said. "But
it's very hard to say which is better."
Many scientists believe that the most elegant way to define the kilogram
is by counting out a kilo's worth of atoms of an element. A project is
under way to test that with gold atoms. But the sheer number of atoms in a
kilogram, a number with roughly 25 digits in it, makes that approach
unfeasible for the foreseeable future.
For now, Dr. Davis is willing to set his sights lower in the error-prone
world of superprecision measurements. "It would be nice," he said, "just
to have two experiments in the world that agreed with each other."
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