page_title: Clock
section_title: Power source
hierarchical_section_title: Clock / Operation / Power source
caption_attribution_description: English: Keys of various sizes for winding up mainsprings on clocks.
context_page_description: A clock is a device used to measure, keep, and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.
  Some predecessors to the modern clock may be considered as "clocks" that are based on movement in nature: A sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of duration timers, a well-known example being the hourglass. Water clocks, along with the sundials, are possibly the oldest time-measuring instruments. A major advance occurred with the invention of the verge escapement, which made possible the first mechanical clocks around 1300 in Europe, which kept time with oscillating timekeepers like balance wheels.
  Traditionally in horology, the term clock was used for a striking clock, while a clock that did not strike the hours audibly was called a timepiece. In general usage today, a "clock" refers to any device for measuring and displaying the time.
context_section_description: 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 clocks, the power source is either a battery or the 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.
caption: Keys of various sizes for winding up mainsprings on clocks
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