| Original Kilo |
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 21.400.000.000.000.000.000.000.000 atoms for one original kilogram.Physicists want to replace the original kilo in Sèvres by a ball made of silicon. Scientists are seeking to replace the original kilogram, the basis for this measure, with a ball that is made of silicon, because the prototype of all calibrated weights across the globe is losing weight, which may indeed be one kilogram, in principle. The scientists of the Physikalisch-Technischen Bundesanstalt in Germany are therefore planning to redefine the kilo by defining the number of atoms in one silicon ball. Since silicon atoms are ordered in a fixed crystal structure, we will have the exact number of atoms. For this end, the ball is measured at 1.6 million points while resting in a climatic chamber at 20 degrees Celsius - the roughness of the surface is caused by single atoms only. When compared to the Earth, these deviations would correspond to about 1.7 meters in height. However, at some times foreign atoms entere the pure silicon and so distort the results. Thus, the ball can be measured with an exactness of only 7 decimal places. The scientists plan to eliminate even this distortion by producing much purer silicon, to get close to the theoretically possible value: 2.14 by 10 atoms are supposed to be in the ball. One advantage of such a definition would be to prevent the ball from changing its weight. The last decimal place of exactitude may be in reach by the development of even purer silicon. In Sèvres at Paris, the safe of the International Bureau for Weights and Measures hosts a cylinder made out of a platinum-iridium composition. All countries have obtained copies of the original kilo to calibrate their scales. But even though they might not quite show the same result, it is the original of all kilos in Paris that is losing weight. |