Redefining the kilogram

Dec. 8, 2005
The kilogram is the last of the seven basic units on the international measurement system still defined by a physical item.

(A 1-kg platinum-iridium alloy cylinder is kept in a vault at the Bureau International des Poids et Mesures near Paris. The other six (meter, second, ampere, Kelvin, mole, and candela) are all defined in terms of nature and can be measured at any suitably equipped lab.

Researchers at the National Institute of Standards and Technology (NIST), a branch of the Commerce Dept., are closer to making a similar definition for the kilogram based on the watt balance.

The device measures the force needed to balance a 1-kg mass artifact against the Earth's gravity, as well as two electrical values. These measurements determine the relationship between mechanical and electrical power, which can be crunched through several equations to define the kilogram in terms of basic natural properties.

Currently, NIST's wattbalance approach has the same precision (0.052 ppm) as the current physical specimen, which is suspected of drifting over the years due to wear and degradation. NIST hopes to simplify the balance and get the precision to 0.02 ppm by the end of next year. This will let the new definition serve commercial mass-measurement needs into the near future, says NIST.

In NIST's watt balance, a kilogram test mass rests on a balance pan connected to a coil of copper wire, which surrounds a superconducting electromagnet. Current sent through the coil creates electromagnet forces that balance the weight of the test mass. The device measures this current and force. The apparatus also moves the coil vertically, and, like an electric generator, this induces a voltage. The velocity and voltage of the coil are also measured. These four measurments determine the relationship between mechanical and electrical power, which can be combined with other basic properties of nature to define the kilogram?

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