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At 125 Years Old, the Kilogram Is Approaching Retirement

The prototype kilogram we measure all mass against will soon be replaced by more constant standards.

by Victoria Turk
Sep 26 2014, 11:15am

After celebrating its 125th birthday this month, the definitive kilogram weight may soon go into retirement. That's not to say we're all going to start using the imperial system, but we may no longer need a block of alloy to define what a kilogram is.

On the occasion of the kilogram's anniversary, the UK's National Physical Laboratory (NPL) explains that the international prototype kilogram—the one block of mass that all other measurements bearing the kilogram name have to measure up to—has been serving as the official global standard since September 1889.

It's pretty simple: the platinum and iridium block sits guarded in the International Bureau of Weights and Measures in France, and sets the standard for what a kilogram should weigh. There are six official copies that are regularly checked against it.

But soon we may have no need for the one kilogram to rule them all. It's been known for a while that the kilogram isn't as constant as we might hope; comparisons between the prototype and its copies have been known to show divergence. At the last check in 1989, there was a difference of about 50 micrograms, which the Engineering and Technology Magazine explains is presumably down to chemical interaction with the atmosphere.

The one. Image: International Bureau of Weights and Measures

As the prototype kilogram is accepted as the "correct" one no matter what, it's impossible to tell how it's changed over time, but on the NPL site scientist Stuart Davidson confirms it definitely will have done. "It is a concern that we know that the IPK [international prototype of the kilogram] must be changing, but there is currently no way to actually measure this change," he says.

But this doesn't mean that the metric system is screwed. The NPL outlines potential methods to standardise a kilogram without the need for an actual physical weight, instead using a constant that's actually constant. And they reckon they'll have a new standard for the kilogram by 2018.

One of these methods, the Avogadro project, involves relating the standard kilogram mass to a fixed number of atoms in a one-kilogram silicon sphere. "It is possible to define the kilogram as a fixed number of atoms of a particular substance, thus relating the kilogram to an atomic mass," the NPL project page explains.

It is a concern that we know that the IPK must be changing, but there is currently no way to actually measure this change

Another centres around the watt balance, an instrument that defines the weight of an object by measuring electrical current and voltage, and that relies on fixing the value of the Planck constant—a fundamental constant in quantum mechanics—to lose any uncertainty.

These kind of unchangeable constants are obviously not subject to change like a physical block, which really remains standard only in name. We already use similar fixed constants for all other measurements, like basing a metre on the speed of light rather than measuring all metre sticks against one definitive metre stick. 

The kilogram is the last physical object used as a standard measure, and at the weighty age of 125, it'll soon be left to crumble in peace.