Explainer #6
How does a quartz watch keep time?
A quartz watch has five steps. A tiny quartz tuning fork rings 32,768 times a second. A simple step that halves the beat is repeated 15 times, which turns that into exactly one beat per second. Each beat sends current through a coil, which flips a small magnet half a turn, and gears move the seconds hand one step.
The principle
1. The crystal: a tuning fork that rings 32,768 times a second. Inside the watch is a tiny tuning fork made of quartz, a few millimetres long and sealed in a little can. Its beat is very steady for three reasons:
- Quartz is piezoelectric. A voltage makes the crystal bend a tiny bit, and when it bends it produces a voltage of its own. That lets a small circuit “push” the fork and “listen” to it at the same time.
- It rings at its own note. Like a tuning fork or a bell, the quartz fork has one natural frequency, set by its shape and size. The circuit only nudges it in time with its own swing, so it keeps ringing at that note. Quartz loses very little energy per swing (engineers say it has a very high “Q”), so the note is extremely pure, hard to pull off pitch, and needs almost no power. The fork is sealed in a vacuum, so air does not damp it.
- The cut barely cares about temperature. The fork is cut from the crystal at an angle where its frequency changes very little with temperature, and it is tuned to be most accurate close to wrist temperature.
2. One halving step. Picture a switch that flips at every beat it gets: on, off, on, off. If you only count the times it turns on, you get one beat out for every two beats in. Each step halves the beat. (Engineers call this step a flip-flop.)
3. Fifteen steps in a row. 32,768 is a power of two: 2¹⁵. So 15 halving steps in a row give 32,768 → 16,384 → 8,192 → … → 2 → 1 beat per second. The frequency is also just above what humans can hear, and a fork that small uses very little power.
4. The coil flips the magnet. Each beat goes to a tiny motor: a coil wound on an iron frame whose two ends sit on either side of a small round magnet. A pulse of current makes one end of the frame a north pole and the other a south pole, and the magnet turns half a turn to line up with them. The next beat sends the current the other way, the poles swap, and the magnet turns another half turn.
5. Gears tick the hand. A small gear on the magnet drives a gear train that moves the seconds hand one step per beat, and onward to the minute and hour hands.
Common myth
The battery sets the pace of the watch
The battery only supplies the energy. The pace is set by the quartz crystal, because the chip counts its vibrations and nothing else. That is why a quartz watch keeps correct time right up until the battery is almost flat. Many watches then warn you by moving the seconds hand in 2- or 4-second jumps, and the makers state that the watch still keeps accurate time while it does this. Soon after, it stops. What does change the rate slightly is temperature: the crystal runs fastest near wrist temperature and a little slower when it is colder or hotter. A typical quartz watch is rated to within about 15 to 20 seconds a month.How this video was made: Rendered 3D model of a simplified quartz movement. The fork's vibration is shown slowed down enormously, the parts are not to scale, and the halving steps are drawn as switches and lights.
