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Inside the High-Tech Lab Where Scientists Are Redefining the Kilogram

The watt balance will tell us what a kilogram is.

by Bryson Masse
Jul 19 2016, 11:00am

Carlos Sanchez of the National Research Council of Canada checks out the watt balance. Image: National Research Council of Canada

At a state-of-the-art lab near Ottawa's downtown core, a team of scientists is working on a device that promises to redefine one single system that connects all of humanity—not religion or creed, but how we measure things in a constant, reliable way.

On Friday, with a group of excited scientists from around the world, I got to pay a rare visit.

In the middle of the National Research Council of Canada (NRC)'s underground lab, surrounded by measurement equipment, was a machine that stood as tall as an average person, and looked like an elaborate balance scale. Its complex nature became clear when I stepped close and my ID badge levitated away from me, drawn by a powerful magnet inside.

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The watt balance measures mass in terms of current and voltage. It's meant to address a dire need in the science of measurement. Barry Wood, researcher at the NRC, explained it to me a couple days earlier, at a metrology convention that was being hosted in the city. The balance will determine "an equivalence between electrical standards and mass standards," he said. In other words, it will finally tell us, in no uncertain terms, what a kilogram really means.

The way we measure things couldn't be more important. Decimal-based fundamental units have united societies around the globe, ensuring that when things like commodities, time, resources, and knowledge are traded, everyone can agree on what is actually there. (And that we know where the pokémon are.) This is encapsulated in the science of metrology.

A goal of metrology has been to tie all of our units of measurement to a constant, so their basic nature stays the same across the universe. For example, the base unit of time—the second—is 9,192,631,770 periods of a kind of cesium atom, and the meter is actually the distance light travels in a specific fraction of a second. Both of these will not change, wherever or whenever you are in the universe.

But what about the kilogram?

Carlos Sanchez of the National Research Council of Canada checks out the watt balance. All photos from National Research Council of Canada

Believe it or not, our current measure of the kilogram is… a lump of metal. In France. Specifically, the International Prototype of the Kilogram (IPK), a platinum and iridium cylinder, is the last physical standard. It has not held up well over the last two decades.

In the late 80s, during a rare calibration, the IPK (or 'le grand K') was found to be slightly lighter than the rest of the copies around the world. This was a massive problem.

"Think of the [IPK] as this seething, boiling thing," said Wood. "It's not this intrinsic thing. It's changing all the time." We need to define it in a way that will be stable forever, and that's what the watt balance will do.

Resting on a yellow pedestal that reaches all the way to the bedrock beneath the city to diminish vibrations, the watt balance looked pretty dang fancy. In the center of one side was a weight placed on a clear Pyrex lattice, which has coils of copper-coloured wire wrapped around its arms and another copper coil to create a drive motor on the other.

The watt balance performs measurements in a very similar way to a classical scale balance (it compares two different forces), but it replaces the pull of gravity with the force of electromagnetism. The watt balance was invented in 1975 by Bryan Kibble from the National Physical Laboratory in the UK. Kibble passed away earlier this year, coming very close to seeing the conclusion to work that's spanned four decades now.

All this effort to pin down the kilogram has had an interesting side effect, Wood explained to me: It's helped scientists redefine Planck's Constant. Physicists are working on reducing the uncertainty of the value (Canada's watt balance measured the most accurate figure in 2014). After two years of taking apart and carefully reassembling the watt balance, they've returned with an even more precise number.

"The final numbers will be settled after July the first, next year," said Wood. "And then hopefully we'll have re-definition in 2018. [After] the fixing of values of fundamental constants."

As for the kilogram, Wood tells me that if all goes well, you and I will never notice the change: 1 kg will still be 1 kg, but a global effort will guarantee that it will be that way forever.