Pocket chronograph watch, c 1900.
© Science Museum / Science & Society Picture Library
This pocket watch is built around trains of gear wheels. A central train connects the mainspring to the hairspring. The hairspring mechanism, invented by Christiaan Huygens (1629-1693) in 1675, spins the balance wheel at a regular rate. One end of the hairspring is fixed and the other end is attached to the centre of the balance wheel. As the spring winds and unwinds, it spins the wheel back and forth.