ARCHIVED - Canada's kilogram weighs in

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May 10, 2011 —

Housed in a vault at NRC is a prized ingot of platinum iridium called K74 ― Canada’s national standard for the kilogram. Rarely does K74 leave home, but this year it travelled all the way to France for an official verification.

Canada is one of the few countries that possess an identical copy of Le Grand K, the International Prototype of the Kilogram. This year’s verification showed that the mass of Canada’s official kilogram has changed slightly since 2001 when it was last compared to Le Grand K at the International Bureau of Weights and Measures.

Canada’s prototype of the kilogram, K74, is secured in its transportation box for the trip to Paris.

Canada’s prototype of the kilogram, K74, is secured in its transportation box for the trip to Paris.

How could Canada’s official kilogram lose or gain weight when it lives quietly under a glass dome in a vault?

K74, Canada’s prototype of the kilogram, resides in an NRC vault.

K74, Canada’s prototype of the kilogram, resides in an NRC vault.

Surprisingly, even when protected from vibrations and environmental influences, all artefacts gradually lose or accumulate atoms over time. This fact worries metrologists because Canada’s international trade relies on an accurate kilogram. Even an infinitesimally small change introduces a margin of error in our trade standards, which can have significant economic implications when multiplied, for example, by millions of kilograms of grain or other commodities.

To ensure absolute precision, Canada’s K74 and the official copies of other nations must periodically be compared to Le Grand K which, by definition, has a mass of exactly one kilogram.

Seeking an immutable standard

“The principle of measurement stability underlies the mandate and value of national metrology institutes around the world," says Dr. Claude Jacques, NRC’s mass standards expert. “That’s why the instability of the kilogram needs to be resolved.”

The kilogram is a vestige of the past. Unique among the base units of today’s International System of Units (SI), it’s the only SI unit still defined by an artefact. And, like Canada’s official kilogram, Le Grand K also gains or loses mass as do the official copies all around the world.

NRC research officer, Dr. Claude Jacques, hands Canada’s prototype kilogram, K74, to the Bureau international des poids et mesures, for verification.

NRC research officer, Dr. Claude Jacques, hands Canada’s prototype kilogram, K74, to the Bureau international des poids et mesures, for verification.

The second and metre have been redefined in ways that allow metrologists to make highly accurate, precise and reproducible measurements all around the globe. When the SI was created, the second was defined as a fraction of the time it takes for the Earth to revolve around the Sun. Now it is defined far more precisely by the natural frequency of a cesium atom. And the metre, first based on the Earth’s circumference, is now tied to the speed of light. These standards are now universal and unchanging, unlike the kilogram.

Driven by concern for greater accuracy and stability in all measurements, scientists around the world are working on new ways to define the kilogram — a crucial element of global measurements in commerce, science and other aspects of everyday life.

NRC has joined the quest to redefine the kilogram based on fundamental physical constants or atomic properties, believed to be both universal in nature and constant in time.

“It has been hugely challenging to redefine the kilogram in terms of a natural phenomenon because it is so difficult to scale mass up from the micro to the macro world,” says Dr. Dave Inglis, head of electrical standards research at NRC. “And, because mass is involved in the definitions of the ampere and the mole, the problem with redefining the kilogram has stood in the way of redefining these other units.”

Redefining the kilogram will be the first step in revamping the International System of Units. And NRC will bring some of the world’s best minds to bear on the task.

The inconstant kilogram

The kilogram is the only international standard unit still defined by an artefact, a fact that worries metrologists.

Why? Because, no matter how well protected, all artefacts gradually lose or accumulate atoms over time. And just a few atoms of change in mass in the kilogram each year can have a huge impact on international measurement and trade.

Related information 

Enquiries: Media relations
National Research Council of Canada
613-991-1431
media@nrc-cnrc.gc.ca

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