For a mechanical clock to work, it needs a source of energy that powers all the components. Spring-driven clocks, for instance, use the elasticity of metals as their source of power. The power unit for spring driven clocks consists of the spring made of a metal ribbon and the barrel that contains the spring. In addition to a power source, a mechanical clock needs a transmission mechanism to transfer the power to the second, minute and hour hands. The complex series of gears found inside a clock is a type of such transmission mechanisms.