ATOMIC CLOCK

Timekeeping devices usually contain or are connected to a machine that swings back and forth or oscillates at a constant rate to control the movement of hands or the rate of change of digits. Mechanical or analog clocks use balance wheels, pendulums and tuning forks as well as quartz crystals as their oscillating machinery for time measurement.
Atomic clocks operate in much the same way - except that they use the frequency of the oscillation of atoms or molecules to measure time - thus the name 'atomic clocks'.
How an Atomic Clocks Works:
A typical atomic clock consists of a cavity in which the core element (usually Cesium-133) is heated to release its atoms. The atoms released have varying electrical charges. The atoms are passed through a vacuum tube then through a magnetic field where only atoms with the correct energy state are allowed to pass through.
The selected low-energy atoms then pass through a concentrated microwave field which is produced by a transmitter controlled by a quartz crystal oscillator that's set to vibrate at 9,192,631,770 Hertz or cycles per second. The frequency of the microwave field isn't always exact and varies from the required vibration, but the variation is always minimal and the correct frequency is cyclically attained.
An atom changes to a high energy state only if it passes through exactly at a time when the microwave field is at the correct frequency. Atoms which have changed energy states are detected and monitored by a device at the end of the vacuum tube.
At this point, another magnetic field sorts and filters the atoms out to identify atoms with the correct energy state. If the atoms counted go below a set threshold level, then the crystal oscillator is not functioning properly and is adjusted so that it is transmitting at the proper frequency. A separate device then converts the oscillation frequency to pulses of exactly one second each.