Voting has ended

First successful atomic clock

world’s most accurate timekeeper London,

A tube suspended in a metal frame supported on a steel base structure, with a caesium source to the left, magnets to deflect the beam at either end and a tuning cavity in the middle.
Caesium atomic clock, 1955. Science Museum/ Science & Society Picture Library

In 1955, Louis Essen (1908-97), assisted by Jack Parry at the National Physical Laboratory in Teddington, built the first atomic clock to outperform existing pendulum and quartz clocks. 

Rival mechanical clocks work by counting the vibrations of something which has a constant frequency, such as a pendulum. Unfortunately, the frequency of a pendulum is affected by changes in temperature, air pressure and the strength of gravity. This causes the clock to run too quickly or too slowly. 

The equivalent of a pendulum can be found in atoms, which are composed of a nucleus surrounded by electrons in different orbits, or energy levels. The electrons can ‘jump’ from one energy level to another and absorb or release energy at a precise frequency. This process completes a certain number of full waves in a given time, similar to the way a pendulum completes a certain number of swings. 

Based on jumps inside the caesium atom, the caesium atomic clock was so accurate that it would only gain or lose one second every 300 years. This eventually led to the second being redefined in terms of atoms rather than the Earth’s motion. Atomic clocks now provide international timekeeping standards and are the basis of GPS navigation systems. 

Science Museum

Engineering, Physics,
National Physical Laboratory, Teddington
Key Individuals
Louis Essen, Jack Parry, National Physical Laboratory,