The coin wound a clock inside the meter
A mechanical parking meter had no battery or electronic timer. The coin did two jobs: it paid for parking and supplied the energy to run a small clockwork mechanism. Inserting it released a latch, allowing the user to turn a knob or handle and wind a metal mainspring—the coiled strip of steel that stores energy when tightened.
Once wound, the spring slowly relaxed. A train of gears translated that gradual motion into the much slower movement needed to measure a parking interval. The meter was, in effect, a tiny mechanical timer bolted to a post.
How time turned into a visible warning
The gears drove a dial or indicator as the spring unwound. When the purchased interval ended, the mechanism moved a red flag into view. That made the meter’s state easy to read from the sidewalk: the flag signaled that the paid time had expired, without requiring an electrical display or a precise reading of the clockwork inside.
The flag was also a practical choice for enforcement. A parking officer could scan a row of meters and spot which ones had run out, while the mechanism itself kept time independently at each space.
A small machine with a built-in limit
Because the timer ran on stored spring tension, the mechanism had to balance two competing needs: the spring needed enough force to keep the gears moving, but it had to unwind slowly and consistently over the parking period. The gears controlled that release, much as the escapement in a wind-up clock regulates its motion.
The result was a straightforward chain of cause and effect: a coin unlocked the winding action, the spring powered the timer, and the gears turned elapsed time into a red flag. Each meter carried its own little clock—and each expired flag was the spring’s final, visible act.
