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Confusing Kilograms

Confusing Kilograms

By Anne Pawsey

A group of hard-thinking scientists are debating the best way to redefine the standard for a kilogram. But surely a kilogram's a kilogram, we thought. Well, it's just not as easy as all that. Anne Pawsey explains all.

If I asked a group of people, how much do you weigh? I expect I'd get an answer in a wide range answers given in a wide range units depending on who I asked; those people who gave their weight in stone/pounds are correct, those who gave it in kilograms in fact gave their mass.

So far so confusing, what's the difference? I'm only going to talk about metric units from now on so the difference is as follows; weight is a measure of force, the attraction between you and the Earth and it's measured in Newtons, mass is a measure of how much matter you are made up from and it's unit is the kilogram.

Weight depends on your mass and the acceleration due to gravity of the environment your in. This is usually the Earth so you don't need to worry about it too much. However if you were to take your bathroom scales to the Moon they would tell you that you weighed a sixth of what you do on Earth. But before all you weightwatchers start rushing for the space shuttle, a word of warning: your mass is exactly the same wherever you are; it's only the attraction due to gravity that's different.


Gran K - a lump of metal in a box, in Paris.
So the important measure is mass, and in standard units we measure mass in kilograms (the imperial measure is the slug). A kilogram is defined as the mass of a lump of platinum-iridium alloy kept in a box in Paris. This is the Gran K and it is one kilogram because the International Bureau of weights and measures says so.

There are other kilograms scattered around the world and every so often they are compared to the original; the last time this was tried none of them matched. This created a bit of a problem, the mass of the metal in the Parisian box keeps changing every so slightly, so the kilogram is also changing but there is no way of telling as the mass in Paris has to be a kilogram.

To get round this problem scientists have been trying to come up with some more high tech alternatives. One proposal is to define a kilogram in terms of the atomic mass. The idea is to create a perfect sphere of silicon and use it to calculate the mass of a single atom, a kilogram could then be defined as the mass of this many silicon atoms. An alternative approach is to use Avogadro’s number (the number of atoms in one mole of substance) to define a kilogram as containing exactly that number of atoms of carbon, the problem is no-one can quite agree on how big Avogadro’s number should actually be.

The final approach is rather complicated and involves quantum mechanics. It turns out the definition of voltage and resistance based on Plank's constant are pretty stable and could be used to define a kilogram using a piece of kit known as a current balance. The kilogram would then be defined as the amount of mass that can be supported by a given amount of current.

Unfortunately none of the approaches have managed to produce results as reliable as the lump of metal in Paris so we are going to be stuck with that for sometime yet.

Read more of Anne's articles or follow the links below:


- Weird - The Brazil Nut Effect
- Cool - How electric eels work
- Interesting - Can you catch a yawn?
- Make your own mind - Genetic modification



Image credit: NRC-CNRC (Harry Turner)



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17 Aug 2010
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