The National Physical Laboratory (NPL) made the first successful atomic clock in 1955. In 1967, the length of a second was redefined as an atomic measurement, rather than an astronomical one. This particular clock, by Hewlett Packard, was used at the NPL in the 1990s for its contribution to the global timescale, called 'UTC'. It was decommissioned in 1999.

Presented by the National Physical Laboratory