The pendulum clock became very popular in the nineteenth and early twentieth centuries. The first pendulum clock was invented by Christiaan Huygens in 1656. This was handmade and consequently not available to the average person. By 1921 Shortt introduced a pendulum clock that was manufactured using the modern industrial production methods of the time.

The pendulum ensures a constant relatively precise movement with the energy of the pendulum passing onto a gear system that drives the hands of the clock.

The first Atomic clock was built in1949. They have been manufactured by governments to give the exact time and are used for scientific research. Consequently they are very expensive and are normally found in research facilities and scientific universities. They work on the principle that molecules and atoms resonate (vibrate) at a stable frequency. The pendulum of a traditional clock moves from side to side and this motion controls a clock mechanism. In the same way the vibration of molecules can be used as the basis to incredibly accurate clocks.
Cesium atoms resonate (vibrate) and this property can be used to tell the time with incredible accuracy. The cesium is heated and as a result atoms boil off and travel down a tube. They pass through a microwave field and their resonance changes. This change in resonance is measured at the end of the tube by a special detector. This allows scientists to measure precisely and to adjust normal clocks (digital etc) so that they are accurate. The most up to date Atomic clocks are accurate to one second