A mechanical watch is a watch that uses a non-electric mechanism to measure the passage of time. They are driven by a spring (called a mainspring) which must be wound periodically, and releases the energy to turn the clock's wheels as it unwinds. They keep time with a balance wheel, which oscillates back and forth at a constant rate, and make a 'ticking' sound when operating. Mechanical watches evolved in Europe in the 1600s from spring powered clocks, which appeared in the 1400s.
Mechanical watches are not as accurate as modern quartz watches and are generally more expensive. They are now kept more for their aesthetic qualities and as jewelry than for their timekeeping ability.
All mechanical watches have these five parts:
- A mainspring that stores mechanical energy to power the watch.
- A gear train, which has the dual function of transmitting the force of the mainspring to the balance wheel, and adding up the swings of the balance wheel to get units of seconds, minutes and hours, etc. A separate part of the gear train allows the user to wind the mainspring and enables the hands to be moved to set the time.
- A balance wheel which oscillates back and forth. Each swing of the balance wheel takes precisely the same amount of time. This is the timekeeping element in the watch.
- An escapement mechanism which has the dual function of keeping the balance wheel vibrating by giving it an impulse each swing, and allowing the clock's gears to advance or 'escape' by a set amount with each swing. It is this part that produces the characteristic 'ticking' sound of the mechanical watch.
- An indicating dial, usually a traditional clock face with rotating hands, to display the time in human-readable form.
Additional functions on a watch besides the basic timekeeping ones are traditionally called complications. Mechanical watches may have these complications:
- Automatic winding or self-winding - in order to relieve the need to wind the watch, this device winds the watch's mainspring automatically using the natural motions of the wrist, with a rotating weight mechanism.
- Calendar - displays the date, and often weekday, month, and year. Simple calendar watches don't account for the different lengths of the months, requiring the user to reset the date 5 times a year, but perpetual calendar watches account for this, and even leap years.
- Alarm - a bell or buzzer that can be set to go off at a given time.
- Chronograph - a watch with additional stopwatch functions. Buttons on the case start and stop the second hand and reset it to zero, and usually several subdials display the elapsed time in larger units.
- Hacking feature - found on military watches, a mechanism that stops the second hand while the watch is being set. This enables watches to be synchronized to the precise second.
- Moon phase dial - shows the phase of the moon with a moon face on a rotating disk.
- Wind indicator or power reserve indicator - mostly found on automatic watches, a subdial that shows how much power is left in the mainspring, usually in terms of hours left to run.
- Repeater - a watch that chimes the hours audibly at the press of a button. This rare complication was used before artificial lighting to check what time it was in the dark.
- Tourbillon - this expensive feature was originally designed to make the watch more accurate. In an ordinary watch the balance wheel oscillates at different rates when the watch is in different positions, causing inaccuracy. In a tourbillon, the balance wheel is mounted in a rotating cage so it will experience all positions equally. The mechanism is usually exposed on the face to show it off.
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