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I have a question about the meter and the kilogram. I know that,
originally, the standard for the meter was 1/10,000,000 the distance from
the North Pole to the Equator. My question is, how was the distance from
the North Pole to the Equator determined in the first place?
My second question involves the Kilogram. Why is a platinum metal cylinder used to measure the mass of one kilogram, instead of the weight of water. It seems like the weight of water would be more exact, etc. Thanks The distance from the equator to the north pole, on a meridian running through Paris yet, was a patriotic choice for the French revolutionaries who originated the metric system. Members of the French Academy of Sciences had determined the shape of the earth to be that of an oblate spheroid earlier in the 18th Century by measuring the length of a degree of longitude at various latitudes, and so the shape and size of the earth was very much a matter of French pride. (See Science for a Polite Society by Geoffrey V. Sutton.) It would have been easiest to assume the earth to be perfect sphere, since it's very easy to measure the length of a degree of latitude and multiply by 90, but that only works if the earth actually is a perfect sphere. The true shape of the earth is an oblate spheroid (with a slight bulge in the southern hemisphere), which means that degrees of latitude have different lengths at different distances from the North Pole, but if the earth's shape is not irregular (as it is now known to be) this can be accounted for; and since Frenchmen had determined the shape of the Earth the originators of the meter took that into account. Unfortunately, according to the history of the meter maintained by NIST, they were off somewhat, so that the length they measured was short by 0.02%. Because of this, the meter later became the distance between two marks on a bar of platinum-iridium alloy measured at the melting point of ice, and in 1960 it was re-re-defined as a certain number of wavelengths of red light from krypton-86. Today it's defined in terms of the speed of light and the second. The kilogram is defined as the mass of a standard object because the mass of a particular volume of water varies too much. Between freezing and boiling the density of water varies by almost 5%, and that's just at one atmosphere pressure! One would have to define a certain volume of water at a particular temperature and pressure. That shouldn't deter us, except that a definition in terms of a particular quantity of water uses the same principle as a definition in terms of a particular quantity of platinum-iridium alloy, and the metal kilogram is a lot easier to handle. What we'd like is a definition, like that of the meter or the second, in terms of an unchanging constant of nature. One of the holy grails of experimental gravity studies is to define the gravitational constant closely enough that it can be used to define the kilogram. The best recent determinations still differ by up to 50%. (See the December 18, 1998 issue of Science.) More information on the definitions of units can be found at this NIST web site.
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