Measuring the World:
From Material to Ethereal

LOCKED in a vault in Paris is a cylinder about the size of a plum. Its mass is exactly one kilogram. It is the kilogram.

For 116 years, this cylinder made of platinum and iridium has been the world's defining unit of mass. It's an easy concept to understand.

Scientists at the National Institute of Standards and Technology in Gaithersburg, Md., announced last month significant progress toward supplanting this cylinder. Their concept is not so easy to understand.

It's a two-story-tall contraption that looks one part Star Trek, one part Wallace and Gromit. Briefly put, it measures the power needed to generate an electromagnetic force that balances the gravitational pull on a kilogram of mass.

"It's such a very complicated thing that's hard to explain," said Richard Steiner, the physicist in charge of the project. He has been working on this "electronic kilogram" machine for more than a decade.

"That's what everybody kind of laughs at," Dr. Steiner said. "They're all impressed it's such a complicated thing and then they ask, 'What do you need it for?' "

The general answer is that humans have always needed to quantify and standardize, to make their world more certain. Without a standard kilogram - roughly 2.2 pounds - how would scientists know their measurements of mass were accurate? Without a standard meter, how would a manufacturer make a ruler and know that it is precise?

More specifically, the high-tech kilogram is needed because scientists prefer a definition based on the universal constants of physics - something they could in principle calibrate in their own laboratories - rather than on an artifact sitting in a distant vault.

Another problem with the kilogram cylinder is that it is not necessarily unchanging. Over time, contamination might add smidgeons of mass, or cleaning might scrub away some atoms, leaving a lesser kilogram. Better, scientists say, not to have to worry about dust, dirt or disaster striking the Paris vault.

The kilogram, in fact, is decades behind the meter, which used to be defined as the distance between two scratches on a metal bar. In 1960, scientists defined the meter in terms of the wavelength of a specific orange light emitted by krypton atoms. In 1983, they redefined the speed of light to be exactly 299,792,458 meters per second, so a meter is now just the distance that light travels in a vacuum in 1/299,792,458th of a second.

The newer definitions hark back to the original metric definitions, which were based on features of the natural world, not human artifacts. A kilogram was the mass of water filling a cube that is one-tenth of a meter on each side, or one liter of volume, and a meter was one ten-millionth of the distance from the North Pole to the Equator, along the path passing through Paris (since it was the French Academy of Sciences that defined the meter).

Neither definition proved practical, and the French scientists botched their calculation of how much the Earth is squashed by the centrifugal force of its rotation, so the metal bar they made to represent a meter was off by a fraction of a millimeter.

It is also not easy to measure precisely a liter of pure water, which is complicated by impurities and gases dissolved in the water and by how water density changes with temperature and pressure. Instead, that platinum-iridium cylinder was established as the official definition, in 1889.

The search for standards began with the rise of civilization. Measures were needed, especially for commerce. At first, people simply used parts of the body. A cubit, for example, was the distance from the elbow to the tip of the middle finger - which differed from person to person, until an Egyptian pharaoh declared a cubit to be the distance from his elbow to the tip of his middle finger (and possibly the width of his palm).

It was hardly convenient to borrow the pharaoh's arm to measure a bolt of cloth, so a piece of granite was carved and declared the official cubit. Other people would make their own cubit rulers, usually out of wood, based on the granite standard.

The same idea underlay the standards for the kilogram and the meter - a cylinder and a bar, respectively. "Those were not bad standards at the time," said John L. Hall, a scientist at the Institute of Standards and Technology and a winner of this year's Nobel Prize in Physics, who helped refine the definition of the meter two decades ago. "But they're kind of hard to duplicate and disseminate."

Dr. Steiner's team with its two-story contraption has now fixed the mass of a kilogram to 99.999995 percent accuracy. To satisfy the international body that sets measurement standards, they probably need to raise that last "5" to an "8."