Something that always takes the same time
How Does a Clock Know When a Second Has Passed?
Every clock is built around one thing that repeats at an unchanging rate, and a way of counting the repeats.
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Step 1 of 5
A pendulum swings at a rate you cannot argue with.
Its period depends on its length and almost nothing else. Give it a bigger push and it swings wider, not faster — which is exactly what a clock needs.
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Step 2 of 5
A gear train counts the swings.
Each swing lets a toothed wheel move on by one tooth. Wheels of different sizes divide that down until one turn means a minute, then an hour.
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Step 3 of 5
A quartz crystal does the same job far better.
A tiny fork of quartz shivers when electricity is applied, and its shape fixes the rate. It is not affected by being carried around, and it is cheap.
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Step 4 of 5
Its rate is chosen to be easy to halve.
Watch crystals shiver 32,768 times a second, because that is two multiplied by itself fifteen times. Halving the count fifteen times gives exactly one tick per second.
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Step 5 of 5
The most accurate clocks count atoms.
A caesium atom absorbs one exact frequency of microwave. Tuning to that frequency and counting the waves defines the second itself.
A good pendulum clock drifts about a second a week. A quartz watch drifts a second a month. A caesium clock drifts a second in 100 million years.
The short version
A clock works by finding something that repeats at an unchanging rate and counting the repeats — the display is the easy part.
Try it yourself
Hang a key on a 25 cm string and swing it. It takes about one second per swing whether you nudge it gently or hard.
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