English: Ammonia maser frequency standard built 1949 at the US National Bureau of Standards (now National Institute of Standards and Technology) by Harold Lyons and associates. This was technically the first atomic clock, although it was not even as accurate as existing quartz time standards. It consisted of a quartz crystal oscillator locked to the 23.8 GHz microwave absorption line of the inversion transition in the ammonia (NH₃) molecule produced by an ammonia maser, with a set of dividers that produced a 50 Hz output signal. Two versions of the clock were built with accuracy of 10^-8 before the approach was abandoned to concentrate on cesium clocks. The oscillator wasn't actually used to run a clock; the electric clock visible at top is a prop. The 8 meter (27 ft) waveguide absorption cell filled with ammonia gas can be seen wrapped around the clock. On the right is the inventor Harold Lyons. On the left, holding a model of the ammonia molecule, is Edward Condon, director of NBS.