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A clock that survives being carried

Why Can a Watch Work on a Moving Arm?

Every good clock for three hundred years was a swinging weight that needed gravity and stillness. A watch has neither, and keeps time anyway.

A pendulum clock Needs gravity, andneeds to stand stillA watch Works upside down, ina pocket, at a run
The same job, and almost nothing in common.

  1. Step 1 of 5

    A pendulum is useless on a wrist.

    Tilt it, carryit, or jolt it−And the swingstops beingregular

    It swings at a steady rate because gravity pulls it back, so the rate depends on gravity pointing the right way and on nothing else pushing it. Tilt it, or walk with it, and the timekeeping is simply gone.

  2. Step 2 of 5

    So a watch swings a wheel against a spring instead.

    1 A hair-finespring pulls thewheel back2 The wheelovershoots andis pulled back3 Five or tenswings a second,in any position

    A small wheel is wound back and forth by a hair-fine coiled spring, several times a second. The spring does the pulling that gravity used to do, so the whole thing works at any angle, in any position, and gravity is not consulted.

  3. Step 3 of 5

    The escapement is what does the ticking.

    One tooth is released The wheelswings past And collects anudge to keepit goingOnetick

    One small part has two jobs. It lets the gears advance by exactly one tooth per swing, and on the way past it gives the wheel a tiny shove to replace what friction stole. That shove and that release together are the tick you can hear.

  4. Step 4 of 5

    Your arm winds it.

    Every movement ofyour arm Turns a weightthat winds themainspring

    A mechanical watch runs on a coiled mainspring, and a weighted half-disc inside spins a little whenever you move. Wearing it is the winding, which is why one left in a drawer stops and one worn daily does not.

  5. Step 5 of 5

    And a cheap quartz watch beats an expensive mechanical one.

    A fine mechanical watchA few secondsout per dayA cheap quartz watchA few secondsout per month

    A quartz crystal shivers 32,768 times a second — thousands of times faster than any balance wheel, and far steadier. So a supermarket watch keeps better time than a hand-built one costing a thousand times more. People buy the mechanical one for the craft, knowing this.

    32,768 is not a random number. It is two multiplied by itself fifteen times, so fifteen simple halving circuits turn it into exactly one tick per second.

The short version

A watch works while being carried because its timekeeper is a wheel on a spring rather than a pendulum, so gravity and jolts no longer set the rate.

Try it yourself

Hold a mechanical watch to your ear and turn it over. The ticking does not change — do that to a pendulum clock and it stops.

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