How does a quartz watch keep time?

A quartz watch keeps time by counting vibrations. A battery energizes a tiny quartz crystal, which vibrates at a very stable frequency, usually 32,768 times per second. A circuit counts those vibrations and sends one pulse per second to a small motor, which turns gears that move the hands. As NIST explains, clocks in general work by counting a periodic event with a known frequency, and the quartz watch applies that idea with a crystal in place of a pendulum. [1] [2]

Quartz is silicon dioxide arranged as an orderly crystal, unlike the untidy atoms in sand. It is piezoelectric: pressure on it produces a voltage, and a voltage applied to it slightly deforms it. When an alternating voltage is applied, the crystal changes shape back and forth, which means it vibrates. The Chinese University of Hong Kong physics page says this vibration is very stable and almost independent of temperature. Hodinkee adds that temperature is still one of the biggest causes of frequency drift, and that the most accurate quartz movements usually include temperature compensation. Most watch crystals are cut in a tuning fork shape. [3] [4] [5]

The main parts are a battery, the crystal, an integrated circuit, a stepping motor and a gear train that turns the hands. The circuit counts the crystal's oscillations and divides the frequency down. Because 32,768 is the 15th power of 2, the division is simple, and Hodinkee notes that the display advances by one second each time the count reaches 32,768. Each pulse turns the motor's permanent-magnet rotor through 180 degrees, always in the same direction. The rotor drives a series of wheels, one of which carries the second hand, and further wheels turn the minute and hour hands. This is why a quartz watch's second hand moves in one-second steps, whereas a mechanical watch's second hand sweeps more smoothly. [6] [4] [5] [3]

Dividing a high frequency down to a low one improves accuracy. NIST gives an example: if a quartz crystal is 0.1 Hz off, the watch loses about 0.26 seconds a day, whereas a pendulum half-swing that is 0.1 Hz off would be wrong by a second in ten seconds. A typical mechanical balance beats at around 2.5 Hz, against 32,768 Hz for quartz. Quality varies widely, though. Hodinkee says the cheapest quartz movements may be good only to about 15 seconds a month, while the Citizen Caliber 0100, with a crystal at 8,388,608 Hz, is accurate to about one second per year. It also says a very good mechanical watch can beat a poor quartz one. [1] [5] [4]

Epson's history notes that early quartz timepieces needed crystals held at a constant temperature, and that falling power use let the standard frequency rise from 8,192 Hz to 32,768 Hz, which is now standard for most devices using quartz oscillators. [2]