Leaping into the '90s with new constants.

Leaping into the '90s with new constants

The last second of 1989 and the first of 1990 were humdingers, at least for people who worry about trillionths of seconds and millionths of volts. For the fastidious keepers of atomic clocks, 1989's last minute lingered for a leap second. They injected the extra second to keep the world's most accurate clocks in step with Earth's subtly slowing spin. Although many might view the exercise as temporal nitpicking, such recalibrations can mean a world of difference for scientists involved in such projects as tracking nuclear missiles or spacecraft.

As champagnee corks sailed past partygoers during the premier second of 1990, water's boiling temperature officially dropped from the customary 100degreesC to 99.97 degreesC. Although it still takes the same amount of energy to bring a cup of water to a boil, scientiests have slightly redrawn the scale they use to measure the temperatures corresponding to such physical phenomena. The International Committee of Weights and Standards agreed last year that 1990 would usher in a new temperature scale with values that correspond more closely with true thermodynamic temperatures (ideal measures independent of any scale), says physicist Barry N. Taylor, who coordinates work on fundamental constant at the National Institute of Standards and Technology in Gaithersburg, Md. Since a point on any temperature scale only approximates the thermodynamic temperature at which a physical even such as the melting of gold occurs, ever-more-precise data pertaining to such phenomena allow scientists to calibrate their scales with less uncertainty.

The way scientists and engineers measure electrical quantitites such as voltage and resistance also changed slightly on New Year's Day. Before 1972, the standard reference for voltage measurements was based on electrochemical cells maintained under specified, but never entirely repeatable, condition. Resistance standards were based on precision wire wrapped around resistors immersed in a temperature-controlled oil bath. The actual electrical behavior of such reference systems depends on real experimental conditions such as fluctuating temperatures and the age of the system's materials. For a more reliable voltage referece, some scientists i 1972 started switching to the constant ratio between a Josephson junction's easily measured frequency and its voltage, Taylor notes. Like the speed of light, this constant remains invariable under different experimental conditions -- a feature that makes it superior to earlier, more fickle reference systems.

A similarly universal quantity known as the quantized Hall resistance has more recently become available as a standard for the ohm, the unit of electrical resistance. By international agreement, scientists around the world have now begun using the same updated values of the two constants. In the United States, this means the volt's value has increased by about 9.26 parts per million; the ohm got a more modest hike of 1.69 parts per million, comparable to adding about an inch to a 10-mile run.

The stainless steel kilogram standards used for legal purposes by the U.S. Department of Commerce have undergone a similar fine-tuning. Careful recalibration against the so-called international prototype of the kilogram -- a lump of platinum-iridium alloy that sits in a vault at the International Bureau of Weights and Measures in Sevres, France -- showed the U.S. legal standards were about 0.17 milligrams overweight. Mass stands out as the last physical parameter of the International System of Units that still rests on a specific artifact. "It would be nice to have some atomic standard of mass" that would be independent of any particular object, Taylor says. He notes that the master kilogram in France remains vulnerable to slight changes in mass due to intermittent cleanings or shedding of atoms.