Nations to weigh the meaning of a kilogram

If things go as planned, by the end of this week, the world will have a new definition of the kilogram.

The change will not require any adjustments to bathroom scales, or alter the heft of a bag of potatoes. But the milestone will serve to make our universal unit of mbad more universally.

In short, instead of basing it on a precious metal locked in a vault in France, scientists have decided to recast the kilogram of something truly immutable, tied by a mathematical umbilical cord to the fundamental constants of nature that have endured since the Big Bang banged.

For those who play in the rarefied world of high-precision metrology – the science of measurement – it does not get any better.

"I'm always saying this is the top metrologist for your country," said Alan Steele, who plays that role for Canada as director-general of the National Research Council's Metrology Research Center in Ottawa.

Dr. Steele is set to cast Canada's vote to adopt the new kilogram definition on Friday.

The kilogram was originally devised as part of the metric system, a byproduct of the French revolution that sought to break the tradition of the pound – an arbitrary amount that harks back to the Roman libra – in favor of more scientifically derived units.

Initially, the kilogram was defined as a mbad of water point. But by 1889, the countries who were then part of the General Conference on Weights and Measures agreed that the value of the unit mbad was to be increased.

They settled on a reference weight machined out of a platinum-iridium that has served the world's prototype kilogram ever since.

This goal has been used in the past, but Canada has played a key role in the production of drugs.

"It's something we're really proud of," Dr. Steele said. "Metrology is about credibility and demonstrating you're good as you say you are."

What Canada turns out to be at the level of an exceedingly small number, known as Planck's constant, which serves as the fundamental increment of action in the universe. In theory, the constant emerges whenever you divide the energy in a particle of light by its frequency. In practice, it's not so easy to measure. Right now, Canada holds the record for the lowest volatility – about 9.1 parts per trillion.

The device is made possible by the Kibble balance, a supercharged version of the standard laboratory scale. But instead of comparing the mbades of two objects, the Kibble balance very precisely sets the mbad of one object against the magnetic force generated by an electric current flowing through a coil of wire. The ingenious design bridges the mechanical realm with the electromagnetic, and effectively allows the kilogram to be bound to Planck's constant for all time.

This has long been a goal of scientists who serve as the arbiters of measurement. The meter and the second have been defined by physical constants, such as the speed of light, for decades. Planck's constant, the kilogram has been a holdout until now.

"It was only in the last two years that it became clear that we could vote for the redefinition," said Michael Stock, director of physical metrology for the International Bureau of Weights and Measures in Sevres, France. The effort, he said, is not driven by the needs of today.

Canada first got into the kilogram game in 2009 when it took over in Kibble balance from Britain and, in Dr. Steele's words, "went to town and made almost every aspect of the experiment better and better."

Planck's constant and, by extension, the kilogram. The workability allowed Canada's blue-chip measurement to be included with a handful of other labs

Carlos Sanchez, who was part of the team that led the work at the NRC, said that the challenge was not just about being more specific but relentlessly beating down a dozen sources of uncertainty in the equipment for several years to get cleanest possible result.

"It takes patience," Dr. Sanchez said. "You have to have a plan, otherwise you can spend your life doing experiments that lead nowhere."

Happily, the NRC's experiments to the Palace of Versailles, where representatives from 60 countries have badembled to the new definition, not only for the kilogram but also for the units of electric current (the ampere), temperature (the kelvin) and particulate quantity (the mole). A unanimous vote is expected, after which the global edifice of measurement,

Which is precisely the point, Dr. Steele said. At the end of it all, "you want the kilogram to still weigh a kilogram."