How do quartz clocks and digital watches keep time?

1. The second hand jumps once every second, all day and all night. Nobody winds this clock. So what tells it exactly when to tick?
2. Behind the face is a small box. A battery pushes electricity through thin metal paths to a tiny computer chip. Without the battery, nothing moves.
3. Inside a little metal can is a sliver of quartz crystal shaped like a tuning fork. Electricity makes it bend back and forth, always at the very same speed: 32,768 times every second.
4. The shivers are far too fast to move a clock hand. So the chip counts them. Each time it has counted 32,768 shivers, exactly one second has passed, and it sends out one pulse of electricity.
5. The pulse runs through a coil of wire, which turns it into a magnet for a moment. That flips a tiny magnet wheel half a turn. Gears pass the turn along, and the second hand moves one step.
6. A digital watch has the same crystal and the same counting chip. It just skips the motor and gears. Each pulse adds one to the seconds shown on the screen.

This is the little box behind the clock face. Tap each part to see it work.

• Battery. The battery pushes electricity around the clock. It needs so little that one battery lasts a year or more.
• Quartz crystal. A sliver of quartz shaped like a tuning fork, sealed in a little metal can. Electricity makes it vibrate 32,768 times a second, always at the same speed.
• Counting chip. A tiny computer chip keeps the crystal shivering and counts every shiver. After 32,768 of them, one second has passed, and it sends out one pulse.
• Coil. Thousands of turns of very thin copper wire. When a pulse of electricity flows through it, the coil becomes a magnet for a moment.
• Magnet wheel. A tiny wheel that is a magnet. Each time the coil switches on, it pushes and pulls the wheel half a turn.
• Gears. Small gears pass the turning along and slow it down, so each flip of the magnet wheel moves the second hand just one little step.
• Hands. The thin second hand takes 60 steps to go all the way around. Then the minute hand has moved on by one minute.

Put the battery in, then change how many vibrations the chip counts before each tick. Compare your clock with the true time.

Experiment: What happens if the chip sends a pulse after counting only half as many vibrations?

1. Tap the battery to put it in and watch both clocks tick together.
2. Choose "Half as many" and compare your clock with the true time.
3. Now choose "Twice as many" and compare again.

What we found: The crystal always shivers at the same speed, so the count decides how long a tick is. Count half as many and each tick comes twice as soon, so the clock runs fast. Count twice as many and it runs slow. Only 32,768 gives exactly one second.

Turn on JavaScript to try it yourself.

This clock is broken. Fix it so that it shows exactly one minute when a true minute has passed.

Every clock needs two things: something that repeats at a steady beat, and something that counts the beats. An old pendulum clock uses a swinging weight for the beat. A quartz clock uses a tiny piece of quartz, which is a hard, clear crystal. Sand is mostly made of quartz.

Quartz has a special trick. If you squeeze or bend it, it makes a tiny bit of electricity. And if you give it a tiny bit of electricity, it bends. So the chip in the clock gives the crystal little electric nudges, and the crystal shivers back and forth like a tuning fork that never stops ringing. It shivers 32,768 times every second, and it is very hard to make it go faster or slower. That is why quartz clocks keep good time.

No clock hand could move that fast, so the chip counts. Every time it has counted 32,768 shivers, it knows that one second has gone by, and it sends one pulse of electricity to a coil. The coil becomes a magnet for a moment and flips a tiny magnet wheel. Gears carry that little turn to the second hand: tick!

Our comparison with a friend counting jump-rope turns is not perfect. A person gets tired and loses count. The crystal never gets tired, and the chip never loses count. A quartz clock is usually wrong by only about 15 seconds in a whole month.

Key words

Quartz

A hard, clear crystal. A tiny piece of it keeps the beat in most clocks and watches.

Vibration

A fast back-and-forth movement, like a ringing tuning fork.

Frequency

How many times something vibrates each second.

Piezoelectric

Able to make electricity when squeezed, and to bend when given electricity.

Pulse

A short burst of electricity. The clock makes one every second.

Frequency divider

The part of the chip that counts vibrations and gives out one pulse for each full count.

Check your thinking

1. Which part gives a quartz clock its steady beat?

   Show the answer

   The vibrating quartz crystal. The crystal vibrates at a very steady speed. The battery only supplies the power.

2. Why does a quartz clock stop when its battery runs out?

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   Nothing pushes electricity to the crystal, the chip and the motor. The crystal needs little electric nudges to keep vibrating, and the motor needs pulses to step.

3. A faulty chip sends a pulse after counting only half of 32,768 vibrations. What does the clock do?

   Show the answer

   It runs twice as fast. Half the count is reached in half a second, so the hand steps twice every second.

4. Why do watch makers choose a crystal that vibrates exactly 32,768 times a second?

   Show the answer

   It is 2 to the power 15, so halving it 15 times gives exactly 1 per second. Simple circuits halve a frequency easily. Fifteen halvings of 32,768 give 1 hertz. A day has 86,400 seconds.

5. How does a pulse of electricity move the second hand?

   Show the answer

   The coil becomes a magnet and flips a tiny magnet wheel, which turns the gears. Electricity in a coil makes magnetism. That pushes the magnet wheel half a turn, and the gears pass it on.

6. What is different inside a digital watch?

   Show the answer

   It has no motor or gears: the pulses change numbers on a screen. A digital watch uses the same crystal and counting chip. Each pulse adds one second on the display.