††††The principle of atomic clocks was first realised by W. Libby in 1946. Atomic clocks are based on the natural vibration of atoms, which are much faster than those of quartz. This enables the clocks to keep time with even greater accuracy.
In 1969, the US Naval Research Laboratory built an extremely accurate atomic clock which used ammonia molecules. It has been calculated that its time-keeping will only vary by one second in every 1.7 million years.
