Tidying up the kilogram.

Preserved as a gleaming cylinder of platinum and iridium, the international standard for the kilogram rests in pristine splendor under glass at the International Bureau of Weights and Measures at Sevres, France. But this isn't good enough for perfectionists.

Stored in air, this cylinder readily accumulates a microscopically thin, but palpable film of "dirt." It's difficult to remove this coating and return the metal cylinder precisely to its original state with each cleansing. Moreover, the cylinder ages in an unknown manner, possibly changing its mass by as much as 50 parts per billion in 100 years. And there's only one. Because of fears that it may be damaged, it can't be used routinely for calibrating the standard kilograms at national measurement laboratories throughout the world.

Metrologists have long dreamed of defining the kilogram in terms of the universally agreed upon and unchanging values of fundamental physical constants rather than by an ill-defined lump of grungy matter. Researchers at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., and the National Physical Laboratory (NPL) in Teddington, England, have now taken a step toward providing one such basis for the kilogram. Both groups use electrical measurements in somewhat different ways to link the mass standard to Planck's constant, a fundamental quantity in quantum physics.

Originally proposed by NPL's Bryan P. Kibble, the technique requires an apparatus consisting of a movable coil of wire suspended in the magnetic field of a strong magnet. Moving the coil through the magnetic field at a certain velocity causes a current to flow in the coil. Researchers measure the velocity of the coil, the current and voltage induced in the suspended coil, and the acceleration due to gravity, From these data, they can derive a mass. Because voltage and resistance can now be expressed remarkably precisely in terms of constants derived from quantum effects involving individual electrons (SN: 1/13/90, p.30), electrical measurements provide a means of defining a mass standard of equivalent accuracy,

So far, neither the NIST nor the NPL researchers have achieved anywhere near the accuracy and reproducibility they would ultimately like to have, but the preliminary results indicate this technique shows promise. "The whole measurement is incredibly clean," Kibble says.

Redefining the kilogram represents more of a tidying up of the international system of units (SI) than a response to a need for an improved mass standard. "At the present time, there's no problem with the current level of accuracy," admits NIST's Barry N. Taylor. "Although the present definition of the kilogram has its problems, [this effort] may be more of an intellectual exercise than a practical necessity"