A mechanical watch is a watch that uses a clockwork mechanism to measure the passage of time, as opposed to quartz watches which function using the vibration modes of a piezoelectric quartz tuning fork, or radio watches, which are quartz watches synchronized to an atomic clock via radio waves. A mechanical watch is driven by a mainspring which must be wound either periodically by hand or via a self-winding mechanism. Its force is transmitted through a series of gears to power the balance wheel, a weighted wheel which oscillates back and forth at a constant rate. A device called an escapement releases the watch's wheels to move forward a small amount with each swing of the balance wheel, moving the watch's hands forward at a constant rate. The escapement is what makes the 'ticking' sound which is heard in an operating mechanical watch. Mechanical watches evolved in Europe in the 17th century from spring powered clocks, which appeared in the 15th century.
Mechanical watches are typically not as accurate as quartz watches,and they eventually require periodic cleaning and calibration by a skilled watchmaker. Since the 1970s, quartz watches have taken over most of the watch market, and mechanical watches are now mostly marketed as a luxury product, purchased for their aesthetic and luxury values, for appreciation of their fine craftsmanship, or as a status symbol.
The internal mechanism of a watch, excluding the face and hands, is called the movement. All mechanical watches have these five parts:
- A mainspring,which stores mechanical energy to power the watch.
- A gear train, called the wheel 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. A separate part of the gear train, called the keyless work, 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. This is the timekeeping element in the watch. Its timekeeping accuracy is due to the fact that it is a harmonic oscillator, with a period of oscillation which is very constant, dependent on the inertia of the wheel and the elasticity of the balance spring.
- An escapement mechanism, which has the dual function of keeping the balance wheel vibrating by giving it a push with each swing, and allowing the watch's gears to advance or 'escape' by a set amount with each swing. The periodic stopping of the gear train by the escapement makes the '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 eliminate 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 the weekday, month, and year. Simple calendar watches do not 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. An annual calendar does not make the leap year adjustment, and treats February as a 30-day month, so the date must be reset on March 1 every year when it incorrectly says February 29 or 30.
- Alarm—a bell or buzzer that can be set to go on 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. This is now a very common feature on many watches.
- 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 originally used before artificial lighting to check what time it was in the dark. These complex mechanisms are now only found as novelties in extremely expensive luxury watches.
- Tourbillon—this expensive feature was designed to make the watch more accurate. It is a demonstration of watchmaking virtuosity.In an ordinary watch the balance wheel oscillates at different rates, because of gravitational bias, when the watch is in different positions, causing inaccuracy. In a tourbillon, the balance wheel is mounted in a rotating cage so that it will experience all positions equally. The mechanism is usually exposed on the face to show it off. The FHH (Fondation de la Haute Horlogerie) definition is: "Any function other than the indication of hours, minutes and seconds, regardless of whether the mechanism is hand-wound or self-winding, mechanical or electronic, and of movement height . The tourbillon is considered complication even if it do not fall within the generic definition." Its function is not to provide additional information, but to adjust the timekeeping even more precisely. It is an adjustment device that is not essentially necessary for the operation of the watch.