Physicist Richard Steiner adjusts the electronic kilogram, an experimental apparatus for defining mass in terms of the basic properties of nature. ©Robert Rathe
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.
Measurements improve prospects for international redefinition of the kilogram
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.
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