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HODINKEE Recommended Reading On Defining The Second

The second is one of the fundamental units of measurement whose definition is zealously safeguarded by the good folks at the International Bureau Of Weights And Measures, outside Paris, and since 1967, the second has been defined by atomic clocks – specifically, cesium atomic clocks. To grossly oversimplify, cesium atomic clocks use the energy from a microwave beam to trigger a transition between energy states with a very precise transition energy, and since 1967, the definition of a second has been defined as, “The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.” In essence, the cesium atom in atomic clocks “ticks” almost 9.2 billion times per second – or I should say, when you count that number of ticks, you know a second has passed.

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