Astonishing Science. Spectacular museum. The first atomic clock,
Caesium I, was designed by Louis Essen and built at the National
Physical Laboratory in Teddington in 1955. Although it was not
the first machine to use atoms for timekeeping, it was the first
to keep time better than the best pendulum or quartz clocks. It
was also the first clock whose timekeeping was significantly
more constant than the rotation of the Earth. Modern atomic
clocks are even more accurate than Caesium I and time is now
defined in terms of atoms rather than the Earth's motion. All
mechanical clocks work by counting the vibrations of something
which has a constant frequency such as a pendulum.
Unfortunately, the frequency of a pendulum is not perfectly
constant. It is affected by changes in temperature, air pressure
and the strength of gravity. This causes the clock to run too
quickly or too slowly. The frequencies measured by atomic clocks
are much higher than those of a pendulum but vary much less, so
atomic clocks keep time much