Editing Clock (section)

===Power source===

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* In mechanical clocks, the power source is typically either a weight suspended from a cord or chain wrapped around a [[pulley]], [[sprocket]] or drum; or a spiral spring called a [[mainspring]].  Mechanical clocks must be ''wound'' periodically, usually by turning a knob or key or by pulling on the free end of the chain, to store energy in the weight or spring to keep the clock running.
* In [[electric clock]]s, the power source is either a [[Battery (electricity)|battery]] or the [[Mains power|AC power line]]. In clocks that use AC power, a small backup battery is often included to keep the clock running if it is unplugged temporarily from the wall or during a power outage.  Battery-powered analog wall clocks are available that operate over 15 years between battery changes.
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