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Message: <blockquote data-quote="Agback" data-source="post: 449003" data-attributes="member: 5328">You are in error. The metre is now defined in terms of the wavelength of a particular colour of light defined in terms of a certain atomic phenomenon. But that isn't how it started out, as you might have guessed from the fact that few of those things were understood in 1797.The metre was originally defined as 1/10,000,000 of the distance from the North Pole to the Equator along the meridian of Paris. Since this was not a convenient standard, a standard metre was constructed out of metal. When more accurate surveys showed a slight discrepancy, it was (sensibly) decided to stick with the standard that had been actually used in measuring things. Later, this standard metre was measured with an interferometer to determine the wavelength standard that we use today (and which any experimentalist can construct in his or her lab with great accuracy).The litre was defined as 1/1,000,000 of a cubic metre, and the gram as 1/1,000 of the mass of a litre of water. Imprecision arose because of the thermal expansivity of water. The kilogram was briefly redefined in terms of the density of water at its triple point, and then was replaced by an arbitrary standard.The mole (that amount of a substance that has a mass in grams equal to the molecular weight of the substance) was not introduced until 1902.So: the convenience of the metric system is a result of its consistent formal structure, not a result of its correspondence with physical fundamentals.Regards,Agback</blockquote>

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