Pendulum-clock

Pendulum’s alternative motion makes hand’s gears rotate, and saves kinetic energy by mechanisms called escapements. The escapement thrusts the pendulum in its opposite direction when it is at the extremity of its route, and makes the cog-wheel advance of one release. When the pendulum arrives to the opposite extremity of its trajectory the process is inverted and the wheel advances of another release. The sequence is repeated until the spring or the weight fall give energy.

Mechanical clock

In a mechanical clock pendulum’s alternative motion is replaced with the balance wheel, which is a set of mechanisms moved by a series of wheels and sprockets through the charge of a spiral spring. In this kind of clock the escapement changes the oscillatory motion in gear rotation, but it can't keep kinetic energy constant like in the pendulum-clock. Therefore the spiral spring, if regularly charged, gives the energy.

Quartz clocks

In the quartz clocks the oscillation is given by a quartz crystal which vibrates in presence of an electric field. The quartz is part of an oscillator circuit which keeps the crystal in constant vibration at its characteristic frequency. By this basic frequency, a next step shows time’s elapse on electronic displays or on a classic hand dial, in a comprehensible way for humans. Unlike mechanical clock, where the energy source is the spring, a quartz clock needs a small battery (pile) which last generally some years.

Atomic clocks

The atomic clock uses the oscillation of caesium-133 atoms. The advantage of using atoms is that they oscillate always speedily and with the same frequency. Moreover, unlike quartz crystals which are subjected to climatic conditions and to sudden changes in temperature, the atoms are not influenced by external environment.