Metric system could get an overhaul

Units such as the kilogram and meter are supposed to be constant. But they are not always. Many of the metric system's basic measurements are based on natural phenomena -- and some may be more variable than people once thought.

“The problem with the kilogram is that it is not possible to determine if the prototype [by which it is defined] changes over time,” said Peter Mohr, a physicist at the National Institute of Standards and Technology. “It seems to.”


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Because of these uncertainties, the international body that oversees the International System of Units, or SI — the old metric system — is considering a proposal to update some of the key definitions in the system to more firmly ground them in the fundamental constants of nature. The proposal would update the definitions of the kilogram, the unit of measure for mass; the ampere, the unit for electric current; the kelvin, the unit for temperature; and the mole, the unit for amount of a substance.

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FRIDAY FEATURE: A history of measurements

The proposal would not change what the kilogram or other units are but would change how they are defined to provide more exact and constant definitions.

The recommended changes were developed by the Consultative Committee for Units, one of 10 advisory committees of the International Committee for Weights and Measures. The committee is expected to submit a draft resolution for the future revision to the General Conference on Weights and Measures, the international diplomatic body that oversees the SI under the Metre Convention, an international treaty of which the United States is a charter signatory.

Before the changes could go into effect, additional experiments are needed at institutes such as NIST to obtain more accurate values for the constants on which definitions would be based, including the Planck, Avogadro and Boltzmann constants. This might be done in four or five years, Mohr said, "but one never knows.”

The SI dates to 1799 when the newly created French republic established a standardized system of scientific measurements to replace arbitrary local systems. This became the metric system, and the Metre Convention of 1875 created the International Bureau of Weights and Measures and established the General Conference on Weights and Measures. The system of international standards over time became a coherent, four-dimensional system with units for electricity as well as space, time and matter, and after the addition of units for temperature and intensity of radiation, the Metric System officially became the International System of Units in 1960.

The SI remains a living system that continues to evolve, Mohr said, and NIST is involved at all levels, with representatives on the various committees and working groups.

“Incidental changes are made every four years,” Mohr said, and more significant changes occur periodically as the science behind them advances. The last significant change was for the definition of the meter in 1983. The meter, originally defined as one ten-millionth of the distance from the North Pole to the equator, in 1960 was based on the wavelength of krypton 86. In 1983 it became “the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.”

Today, the kilogram is the only SI unit still based on a physical artifact, a 130-year-old platinum-iridium cylinder maintained by the International Bureau of Weights and Measures in France. Because its mass appears to change as matter accumulates on it and it is cleaned, the new proposed definition would be based on the Planck Constant, which reflects the size of minimum energy packets in quantum mechanics, along with two other constants.

The updating of standards is "probably a never-end[ing] process,” Mohr said. As technology and the ability to make measurements improve, units of measure will have to adapt. “This is a big step, though, because it is a conceptual change. The ideal is that they all will be defined by values of fundamental constants.”



About the Author

William Jackson is freelance writer and the author of the CyberEye blog.

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