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Physicist
Richard Steiner adjusts the electronic kilogram, an
experimental apparatus for defining mass in terms of
the basic properties of nature.
©Robert
Rathe
For
a high-resolution version of this photo contact inquiries@nist.gov.
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GAITHERSBURG,
MD—A leading experimental method for defining
the kilogram in terms of properties of nature is now more
accurate than ever, scientists at the Commerce Department’s
National Institute of Standards and Technology (NIST) reported
today. The advance may move the scientific community closer
to redefining the kilogram, the only one of the seven basic
units of the international measurement system still defined
by a physical artifact.
The latest
NIST work, described in the October 2005 issue of Metrologia
and published online today,* confirms the institute’s
1998 results using the same method while reducing the measurement
uncertainty by about 40 percent, thanks mainly to improvements
in the hardware used in the experiments.
“The fact
that we got the same values gives us confidence that the uncertainties
we’re quoting are probably reasonable,” says NIST
physicist Richard Steiner, lead author of the paper.
Scientists
at NIST and other institutions around the world have spent
years conducting experiments to find a reliable definition
based in nature to replace the current international standard
for the kilogram, a century-old cylinder of platinum-iridium
alloy about the size of a plum. The new results mean that
the NIST method, using an apparatus called the watt balance
or electronic kilogram, is almost accurate enough now to meet
the criteria for redefinition.
Any decision about
when and how to redefine the kilogram would be made by an
international group, the International Committee for Weights
and Measures, CIPM, and ratified by a General Conference on
Weights and Measures (CGPM), which next meets in 2007. The
CGPM likely will delay a redefinition until other groups confirm
the new NIST results.
The primary kilogram
standard is currently maintained at the Bureau International
des Poids et Mesures (BIPM) near Paris. Although the cylinder
is housed in a special vault under controlled conditions,
its mass can drift slightly over time and can change because
of contamination, material loss from surface cleaning or other
effects. Moreover, the standard is accessible only at BIPM
and could be damaged or destroyed. By contrast, a property
of nature is by definition always the same and can, in theory,
be measured anywhere.
The other
six basic units of the international measurement system are
the meter (unit of length), second (time), ampere (electric
current), kelvin (temperature), mole (amount of substance)
and candela (luminous intensity). All six are defined in terms
of properties of nature and can be measured at any suitably
equipped laboratory.
The NIST
watt balance is a two-story-high apparatus designed to redefine
mass in terms of fundamental physics and quantum standards.
It measures the force required to balance a 1-kilogram mass
artifact against the pull of Earth’s gravity, as well
as two electrical values (see graphic below). These measurements
are used to determine the relationship between mechanical
and electrical power, which can be combined with several equations
to define the kilogram in terms of basic properties of nature.
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In
the NIST watt balance experiment, a kilogram test mass
is placed on a balance pan that is connected to a coil
of copper wire, which surrounds a superconducting electromagnet.
If electric current is sent through the coil, then just
as in an electric motor, electromagnetic forces are
produced to balance the weight of the test mass. The
apparatus measures this current and force. The apparatus
also can move the coil vertically, and, like an electric
generator, that induces a voltage. The velocity and
voltage of the coil also are measured. These four measurements
determine the relationship between mechanical and electrical
power, which can be combined with other basic properties
of nature to redefine the kilogram.
View a high-resolution version of this image.
Image
credit: R. Steiner/NIST |
One of
these properties is the Planck constant, the ratio of the
energy of radiation to its frequency. This is one of an extensive
set of “fundamental constants” used by scientists
to predict a wide range of phenomena. The latest NIST value
for the Planck constant reported in the new paper (6.62606901
x 10-34 joule seconds) is equivalent to the 1998
NIST result and a 1988 measurement by the National Physical
Laboratory in the United Kingdom, which are the two other
most accurate values.
The watt
balance is one of two leading approaches for redefining the
kilogram. The other approach involves counting how many atoms
of a specific atomic mass equal the mass of 1 kilogram. The
latest NIST measurements, which have an uncertainty of 0.052
parts per million compared to 0.087 parts per million in the
1998 experiments, are far more precise than any previous results
by any research group using either approach, according to
Steiner. The total uncertainty is calculated by adding up
more than 20 sources of error.
The precision
of the latest NIST measurements is roughly equivalent to the
suspected drift in the current kilogram standard’s mass
over time, Steiner says. The NIST researchers hope to further
reduce the uncertainty of the watt balance measurements to
0.02 parts per million within the next year or so, to reach
the level of precision needed for commercial mass measurements
in the near future.
The measurements
reported in Metrologia are the product of numerous
improvements in NIST’s watt balance, including reconstruction
of most of the hardware to eliminate many sources of error.
The coil and balance were enclosed in a fiberglass vacuum
chamber, which reduced the need for air corrections. In addition,
the coil was stiffened to reduce flexing that caused excess
“noise” in the signals being measured. Noise was
reduced to one-fifth of the level of the 1998 experiments.
Researchers also improved the alignment of instruments, temperature
control and the software used for management and analysis
of the experiments.
As an agency of
the U.S. Department of Commerce’s Technology Administration,
NIST develops and promotes measurement, standards and technology
to enhance productivity, facilitate trade and improve the
quality of life.
* R.
Steiner, E.R. Williams, D.B. Newell and R. Liu. “Towards
an electronic kilogram: an improved measurement of the Planck
constant and electron mass.” Metrologia. 42
(2005) 431-441. Published online Sept. 13, 2005.
For additional background on efforts to re-define the kilogram
in terms of natural properties see: http://www.nist.gov/public_affairs/newsfromnist_redef_kilogram.htm
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