Frequency reference masers use glowing chambers of ionized gas, most often Cesium, but when scientists want to tune the Cesium clocks, they use an even more precise Rubidium clock that costs more to run.
A microwave radio source fills the chamber with a standing wave of radio waves. The Cesium atoms absorb the radio waves and emit light.
A photocell looks at the light. When the light gets dimmer, electronics between the photocell and radio transmitter adjusts the frequency of the radio transmitter. This adjustment process is where most of the work and complexity of the clock lies. When a clock is first turned on, it takes a while for it to settle down before it can be trusted.
A counter counts the waves made by the radio transmitter. A computer reads the counter, and does math to convert the number to something that looks like a digital clock, or a radio wave that is transmitted. Of course, the real clock is the original counter.
The National Institute of Standards and Technology maintains atomic clocks for public access.
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