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How many basic standards do we need? Standards are necessary to measurement, and for reasons of accuracy and convenience many measurements involve frequency. With atomic and molecular transitions serving as references, measurement precisions near are possible. The duration of the second is determined by a resonance in the cesium atom, and international atomic time is the reference for all time and frequency measurements in the world. Furthermore, frequency measurementslead,via the speed of light, to the measurement of wavelengths and, viatransducers, to the measurement of many other physical quantities such as temperature and pressure. As a result, time and frequency metrology is at the root of any thinking to revise or improve our system of basic standards of measurement.
Perhaps all standards—including those of length, mass and temperature—can be redefined by linking them to the most accurate standard, that of time, which is known to an accuracy of