In 1948, the first atomic ammonia clock was invented at the US National Bureau of Standards but was less accurate. In the year 1955, Louis Essen at the National Physical Laboratory in the United Kingdom built the first caesium standard based accurate atomic clock.
In the 1950s, the National Company manufactured the first commercial atomic clock known as Atomichron. An atomic clock is designed to measure time according to vibrations within atom and is the most accurate type of timepiece in the world.