English: Mainspring in the movement of a cheap 1950s alarm clock, the Lux Apollo. Shown in an unwound state. The outer end of the spring is attached to the post at bottom. Turning the key at top (normally projecting from the back of the clock) turns the arbor, winding the mainspring more tightly around it. Then the force of the mainspring turns the arbor in the other direction, turning the clock's gears.
WARNING - Never disassemble a clock or watch with a mainspring without the proper tools and techniques. The mainspring contains a lot of energy and can release suddenly, causing serious injury.