![]() ![]() The frequency of this atomic clock is in the microwave region of the electromagnetic spectrum and is a convenient one for locking a microwave oscillator. Prior to 1964 the international standard second had been based upon the orbital period of the Earth, but the cesium clock period was found to be much more stable than the Earth's orbit! The SI unit of time, the second, is now defined by this transition in cesium. In 1967 a standard second was adopted based on the frequency of a transition in the Cs-133 atom:ฤก second = 9,192, 631,770 cycles of the standard Cs-133 transition The current time standard for the United States is a cesium atomic frequency standard at the National Institute of Standards and Technology in Boulder, Colorado. Atomic clocks are integral parts of the Global Positioning System since extreme accuracy in timing is necessary for the triangulation involved. Such clocks have provided the accuracy necessary to test general relativity and to track variations in the frequencies of pulsars. ![]() The two most widely used atomic clocks in recent years have been the cesium beam atomic clock and the rubidium clock. The frequencies associated with such transitions are so reproducible that the definition of the second is now tied to the frequency associated with a transition in cesium-133: 1 second = 9,192, 631,770 cycles of the standard Cs-133 transition Very accurate clocks can be constructed by locking an electronic oscillator to the frequency of an atomic transition. ![]()
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