How long is a second? It’s 9,192,631,770 cycles of caesium radiation, according to today’s International Standards, which are based on Louis Essen’s atomic clock!
Born #onthisday in 1908, Essen, in collaboration with scientist Jack Parry, designed and built the world’s first accurate caesium atomic clock in 1955. It used conventional quartz crystal oscillators, which made it highly accurate compared to the first generation atomic clock that was based on an ammonia maser device. The new clock will only lose around one second over the course of 300 years.
Twelve years after Essen and Parry’s new clock was built, it became the basis for the internationally agreed definition of a second, shifting the standard from astronomical time to atomic time. And this is how a second is still defined today.
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