The H1 clock is a hydrogen maser atomic clock developed by the National Physical Laboratory (NPL) in the United Kingdom. It is one of the most accurate clocks in the world, with an accuracy of one second in 300 million years. The H1 clock is based on the natural vibration frequency of hydrogen atoms, and it uses a microwave cavity to measure this frequency. The clock was first developed in the 1950s, and it has been continuously improved since then. The H1 clock is now used as the primary time standard for the United Kingdom, and it is also used in a number of other countries around the world.