Metrology, the science of measurements, needs reliable measurement units. Starting with the "Meter Convention" in 1875, a worldwide agreement exists for standards and units which guarantee international uniformity. Today, seven base units are used (international system of units, SI units) which form the basis for all measurements. These fundamental units are: the second for time, the meter for length, the kilogram for mass, the kelvin for temperature, the ampere for electric current, the candela for luminous intensity, and the mole for the amount of substance. The General Conference on Weights and Measures, which meets in Paris since 1889 in roughly 4-year intervals, is responsible for the best definitions of base units in respect of accuracy and stability. In this way the meter definition changed during the last 100 years froma prototype (related to the one ten millionth part of the quarter of the terrestrial meridian) to a definition of 1,650,763.73 wavelengths of a special spectral line of the Krypton-Isotope ~(86)Kr to a fixed value of the velocity of light.
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