Density/Talk

Water isn't really defined to have density 1 is it? That's a schoolboy definition. --drj

Mayhaps, I certainly was a schoolboy once :-) The fact that one litre of water weighs exactly one kilogram os no accident however. The SI units were chosen carefully, although i know not if the metre or the kilogram was defined first (my money is on the metre), the relation between them is indeed found through the density of water (at 4 degrees celcius if memory serves correctly). --Anders Törlind

Oh, by the way, is the kilogram still defined as the lump of platinum they hold in Paris, or is it defined as a certain number of atoms of something? --Anders Törlind

I wondered about this recently. My research confirms what kilogram says. There is a standard mass made from platinum/rhodium. I guess it is too difficult to define it as the number of atoms of something. --drj

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I believe that the metere was originally defined to be 1x10<sup>-6</sub> of the distance from the equator to the north pole via a line that went through Paris, this was mesured incorrectly at the time, so the distance is somewhat different. The other units as far as I know are based on that. The kilogram was originally based on a cubic decimeter of water, but that is much too variable to be used as a reference.-- mike dill