First Practical Cesium Atomic Clock

On May 24, 1955, Louis Essen and Jack Parry operated the first practical cesium atomic clock at the National Physical Laboratory in Teddington, UK. The clock used the resonant frequency of cesium-133 atoms, achieving accuracy of approximately one second in 300 years — about 30 times more precise than quartz crystal clocks of that era. The device shifted timekeeping from astronomical observations based on Earth's rotation to atomic transitions governed by quantum mechanics, leading to the redefinition of the second in 1967.