What is Atomic clock?
{{Use mdy dates|date=October 2018}} [[File:Atomicclock.jpg|thumb|An atomic clock]] An '''atomic clock''' is a [[clock]] that works with [[energy level]] of [[atom]]s, as opposed to most other clocks which are [[mechanics|mechanical]]. The [[frequency]] comes from the crossing radiation of [[electron]]s. Atomic clocks are currently the most exact clocks of the world. They are also called ''primary clocks''.<ref>{{cite web|url=http://www.ptb.de/de/org/4/44/441/info3.htm|title=The primary clocks|access-date=December 1, 2009|language=de}}</ref> Most clocks know the time because they count how many times something moves back and forth. Atomic clocks count how many times an atom wiggles back and forth. == Variants == There are several variants of atomic clocks, the most important being beam clocks, fountain clocks, and optical clocks. === Beam Clocks === Beam clocks are the most common kind of atomic clocks. They are also the least exact, but they are still very exact compared to most normal clocks like [[Quartz clock|quartz clocks]] or [[mechanical clocks]]. ==== Timekeeping Atoms ==== The most commonly used atom for atomic clocks is [[Caesium]].<ref>{{Cite journal|date=2009-08-26|title=NIST’s Cesium Fountain Atomic Clocks|url=https://www.nist.gov/pml/time-and-frequency-division/time-realization/cesium-fountain-atomic-clocks|journal=NIST|language=en}}</ref> Specifically Caesium-133, because when you put Caesium-133 in a beam of [[Microwave|microwaves]], the atom wiggles very fast, and keeps wiggling at that speed for a long time. The atoms wiggling very fast means that it can keep time very well. ==== Tuning ==== The Caesium-133 atoms have a resonant frequency, which is the speed that they wiggle at. When the beam of microwaves has the same frequency as the [[resonance]] frequency of the Caesium atoms, then they will all start wiggling. To get the microwave beam to the resonance frequency, the Caesium atoms are hit by the microwaves, and then measured with a [[laser]] or another microwave beam to see how many