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Due to the quantized energy levels of allowed electron orbitals, single atoms can easily absorb energy around certain narrow wavelengths. A cesium atom has one of these narrow absorption bands at a frequency of 9,192,631,770 Hz. A cesium clock can produce EM radiation in this region (microwaves) and detect how well the cesium atoms are absorbing it. So ...


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The kilogram is the base unit of mass because electrical engineers in the late 19th century chose a particular set of practical electrical units. These practical units were a success, and we are still using them today: ohm, volt, ampere, and the joule. In 1874 the mechanical units cm, g, s ('CGS') were adopted as the coherent system of units for science. ...



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