The secret world of the kilogram

Not the international standard, this image is of the copy assigned to the United States in 1889. It is occassionally recertified to ensure it is accurate.

In the earlier days of the metric system, or the SI system as it is properly called (short for Systeme International d’Unites, or International System of Units, if you couldn’t puzzle through the translation yourself), standards were established using physical objects.

Many have heard the story of the calculation of the meter — an erroneously calculated 1/10,000,000 of the distance between the North Pole and the equator — which was then cast as a brass bar to serve as the international standard.  Since that time, the definition of the international standard has been modified in order not to ensure the consistency of distance.  The meter is now defined (since 1983) as the length travelled by light in vacuum during 1/299,792,458 of a second.  You know, because that’s handy to figure out at home.

But this post is about the kilogram, still defined by a small cylinder of a platinum and iridium alloy:

Since 1889, the year the Eiffel Tower opened, that cylinder has been the standard against which every other kilogram on the planet has been judged. But that’s creating problems. According to scientists, the cylinder’s mass appears to be changing.

The original, the one and only standard, is kept locked in a vault.  Copies of the cylinder have been made in order to protect the one true standard, but in the three times that it has been removed from the vault to confirm the copies are true, its weight appears to have changed.

The real crux of this problem is that it’s impossible to tell what has changed over the past 120 years. The copies may have grown heavier over time by absorbing air molecules. But it’s equally possible that the kilogram is getting lighter. Periodic washings, for example, may have removed microscopic quantities of metal from its surface.

The fascinating aspect of this story is the idea of one physical object being the sole benchmark by which an international system is determined.  What would happen if that small cylinder was stolen?  Instantly, no one would know for certain what they weighed.  Conflicts over the amounts of goods bought and sold at the industrial level would explode.  It would be a catastrophe of some weight.

Current work is being done to find another number — a true constant, similar to the example of the meter — in order to replace the cylinder.  A good idea, to be sure, for those involved in scientific work where excruciatingly small weights make a difference.

For the rest of us, it is the sterilization of a romantic idea.