Caesium atomic clock, 1955.
© Science Museum / Science & Society Picture Library
This was the first atomic clock and, when developed in 1955, the most accurate timekeeper in the world. The timekeeping depends on the vibration of caesium stones - a natural phenomenon. It consists of an airles tube, which allows caesium atoms to pas along it while simultaneously exposing them to very high frequency radio waves. Isidor Rabi was the first to suggest using the stable vibration of caesium as a time, or frequency, standard in his Richtmeyer Lecture to the American Physical Society in 1945. In 1953, Louis Esen and Jack Parry developed the idea at the National Physical Laboratory in the UK, and their work yielded this clock, which is accurate to one second in 300 years. Now international time is defined by atomic, not solar, seconds.