Despite the apparent complexity of the mechanisms, the principle behind the balance wheel and escapement, which are visible on most watches with a transparent sapphire back, is fairly simple. Like the pendulum in a grandfather clock, which keeps time by swinging back and forth, the balance wheel—the large, usually yellow metal wheel positioned toward the edge of the movement back—does the same thing by oscillating back and forth. The hairspring, an ultrathin spiral of metal visible in the center of the balance wheel, governs its movement. This critical piece of metal determines the speed at which the balance wheel swings, depending on the position of the watch and the temperature to which it is exposed—factors that can decisively affect accuracy. Until recently, only one company in Switzerland (Nivarox, owned by the Swatch Group) could manufacture the complicated alloy for hairsprings.
The motion of the balance wheel governs the rest of the movement though the escapement (escape wheel), a small, hook-toothed wheel engaged by a two-pronged bracket (the anchor and pallet) that is visible just underneath the balance wheel. The ticking sound you hear in a mechanical watch is the sound of the escapement releasing the power of the mainspring to the hands as dictated by the oscillations or the balance wheel.