The term atomic clock is the general name used to describe any variety of time keeping
devices based on the regular vibrations associated with atoms.
One of the first atomic clocks or as it is also known the ammonia clock. Was developed by
the National Bureau of Standards, and was based on the measurements of the vibrations of
atoms of nitrogen, oscillating back and fourth in ammonia molecules at a rate of 23,870
vibrations per second.
The modern day atomic clocks are based on caesium atoms. The spectrum of caesium includes
a feature corresponding to radiation with a very precise frequency of 9,192,631,770 cycles
per second. One second is now defined as the time it takes for that many oscillations of
the radiation associated with this feature in the spectrum of caesium. This type of clock
is known as a Caesium clock and it is accurate to one part in 10,000 billion, or one second
in 316,000 years.
Even more accurate clocks have been developed using radiation from hydrogen atoms. They
are known as Hydrogen Maser Clocks, and one of these clocks, at the US Naval Research
Laboratory in Washington, DC, is estimated to be accurate to within one second in 1.7
million years. In principle, clocks of this kind could be accurate to one second in 300
million years!
