The kilogram hangs in the balance
Canada achieves most precise measurement of one of the most important constants in science
November 16, 2017 – Ottawa, ON — National Research Council of Canada: One year from now, there will be a revised kilogram. That will mark the moment in time when all units within the international system of units are calculated mathematically, rather than being based off artifacts and physical items. This leap in science is due in no small part to the National Research Council’s (NRC) contribution of a calculation of Planck's constant with the lowest uncertainty ever achieved.
Being able to define the Planck constant, and in so doing accurately calculate the actual value of a kilogram, has been a goal of scientists around the world for over 30 years. Working in an underground laboratory of one metre thick concrete walls formerly used for atomic physics research, NRC scientists have measured the Planck constant with a precision of 9.1 parts per billion. This would be equivalent to accurately counting all the hairs on the heads of 1100 people, to within 1 hair.
The team achieved this result with their Kibble Balance, originally purchased from the UK’s National Physical Laboratory in 2008, which balances the weight of an object against an electromagnetic force.
While simple in principle, to achieve the required accuracy meant the scientists had to account for effects that are imperceptibly small. They accounted for changes in gravity due to the motion of the moon, the amount of water in the ground from snow melt, even the weight of a few layers of water molecules that stick to any surface in a humid environment. In fact the balance is so sensitive small earthquakes on the other side of the world could be observed.
Although the concept of weight and accuracy seems relatively straightforward in most day-to-day applications, like buying fruit or weighing shipping containers, there are significant implications on many of the fundamental sciences. As the world’s science community works towards new standards in measurement and a new definition of Planck’s constant, many future innovations may very well be possible because of the NRC’s relentless search for accuracy and their collaborative spirit.
The NRC team's results are now published online and in the June 2017 issue of Metrologia.
“All you need to know to understand about how the Kibble Balance works, you learned with Newton’s principles in high school science.”
– Carlos Sanchez, Team Leader, National Research Council
“While some aspects of every scientific breakthrough can be attributed to luck, our achievement stems from continually improving each and every component through perseverance.”
– Barry Wood, Fellow, National Research Council
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