Mystery solved - oldbloke's mutterings
They changed the definition of a litre a while back, apparently.
Instead of it being the volume of a kilo of water it's now just 1000cc.
Something to do with the standard temp and pressure previously used making a kilo of water more than 1000cc
Or perhaps it's the other way around. the kilo is no longer defined as the mass of a litre of water.
I think the original units started with defining a metre as being 1/10,000,000 the distance from the pole to the equator. The cubic metre then subdivided into 1000 litres, giving a volume measurement. They then took a litre of water, and that gave the original kilogram.
But it is not exactly 10,000 km from pole to equator at sea level, because the measurement depends on exactly which path you follow, so the km had to be defined in a more exact (and usable!) manner.
|Date:||September 8th, 2011 07:20 pm (UTC)|| |
According to wiki, the litre was defined as the volume of a kg of water (at some specified temp&press) and that's what they changed.
But it's only wiki. Or I may have mis-read it - it was late.
|Date:||September 8th, 2011 03:31 pm (UTC)|| |
Must admit, I thought - inasmuch as I thought - that 1L=1Kg purified water at a certain fixed temperature, and that the definitive Kg lump of whatever it is was kept under strict controlled conditions in a museum or lab or something somewhere in France. Such at least is my very vague memory of O Level physics.
That paragraph probably speaks volumes (ha!) about my understanding of things at school.
|Date:||September 8th, 2011 07:45 pm (UTC)|| |
This may be of interest (from the current issue of one of the mags I write quite a bit for)
|Date:||September 8th, 2011 07:58 pm (UTC)|| |
one or two curious carbuncles in the system. The most glaring is the one remaining specific substance that is used in a very fundamental definition, that of the second. Although 133Cs is in principle universally available, it does seem out of the spirit of the new SI to identify a single arbitrary substance as a keystone
Surely the system needs to be tied to the actual physical universe at at least (and probably preferably no more than) one point? Otherwise it's just a big circular calculation, and Excel won't stand for that.