Standards — Why Is a Kilogram a Kilogram?

There used to be an object at the top

For a long time the answer was an object. In a vault near Paris lay a bar of platinum, and the length of that bar was one metre. Beside it sat a small platinum cylinder, and the weight of that cylinder was one kilogram.

Every ruler and every scale in the world was in the end a copy of those two things. Nations took copies home, copied the copies, and sent them out to factories and hospitals and markets.

And then the object changed

After about a century the copies were gathered and compared with the original, and they no longer agreed. Only slightly — far less than a hair. But disagreement is disagreement.

Something from the air may have settled on it; cleaning may have taken a little away. The real trouble came next.

So the object was abandoned

The answer was to move the standard from an object to nature: something that needs no vault, does not wear, and reads the same in any country.

Length went first. A metre is no longer the length of a platinum bar but the distance light travels in a very short time. The speed of light is the same everywhere, so a metre on Mars is still a metre.

Whether it matters that two rulers disagree is something you feel by measuring. On the board below, measure one stick with a ruler whose divisions differ from the next one, read your number aloud, and see what comes back.

Tool Measuring by the Same Stick — Who Decides a Kilogram? Measure one stick with two rulers whose divisions disagree, then read your number aloud and see what comes back. Follow the unit mismatch that lost a spacecraft, and press through the seven base units to see what defines them now. (Korean only)

A little further in

Time from the trembling of an atom

The second went the same way. It used to be a division of one turn of the Earth — but that turn slows a little and wanders. As clocks go, the planet is not conscientious.

A second is now the time a caesium atom takes to tremble an agreed number of times, somewhat over nine billion. Atoms keep the same beat wherever they are, and no clock is steadier. This is the atomic clock.

The last one to be an object

The kilogram held out longest. While other units moved one by one onto constants of nature, the kilogram remained that cylinder in Paris — the last unit in the world tied to a thing.

That ended in 2019. The kilogram now follows from the Planck constant, measured on a balance that sets weight against electromagnetic force: an object on one side, a current through a coil holding it up on the other, and the weight read from how much current it takes.

So mass is now weighed with electricity. A story about scales turning suddenly into a story about electromagnetism is the charm of this unit.

Why it matters

What shakes when a standard shakes?

1. Trade. If the seller's kilogram and the buyer's kilogram differ, nothing can be priced.
2. Manufacturing. Semiconductors work at tens of thousands of times finer than a hair; a ruler slightly off means parts that do not fit.
3. Medicine. In radiotherapy a dose slightly off does harm.
4. Navigation. Finding your position by satellite is really the measuring of time; a clock that drifts is a place that drifts.

Standards are an invisible agreement lying under a society. The times nobody thinks about them are the times they are working.

The top of the chain in Korea

The Korea Research Institute of Standards and Science, in Yuseong-gu, Daejeon, keeps that top. The standard time the country runs on comes from there too.

Scales in hospitals, pumps at petrol stations, instruments in factories — all are checked somewhere. That checking body is itself checked higher up, and following the chain upward brings you to this institute. Which in turn compares itself against the national institutes of other countries, so the chain does not break at a border.

It is unglamorous work. Nothing is discovered and nothing makes the news. But if the chain breaks, every number in the country loses its meaning.

What you would study to do this

Measuring has branches of its own.

1. Someone who builds atomic clocks and makes them better — atomic physics and optics.
2. Someone who measures length with light — lasers and precision optics.
3. Someone who runs the balance that sets weight against current — electromagnetism and precision mechanics.
4. Someone who takes on the awkward quantities, temperature and pressure — thermal physics and instrumentation.
5. Someone who carries the standard down to the world's scales and rulers — calibration and certification.

That last line is the true size of the field. Accuracy that stays inside the institute is of no use. It becomes a standard only when it reaches the butcher's scale down the road.

The question that remainsIf a standard cannot measure itself — against what are we holding the standards we use to judge right and wrong?