How a Quartz Wall Clock Keeps Time

Every tick starts with a shaking crystal

Turn a battery wall clock over. Behind the dial sits a small plastic box, the movement, with a slot for one AA battery. That box decides when every second begins and ends, and it does it with a sliver of quartz that vibrates tens of thousands of times a second.

As NIST, the US national measurement lab, explains it, a quartz clock uses a piece of quartz crystal in an electronic circuit, where it vibrates at a steady rate, usually 32,768 times a second. The circuit counts those vibrations and turns every 32,768 of them into a one-second tick. A tiny motor and a set of gears turn each tick into moving hands.

So the clock has three jobs: keep a steady beat, count it, and move the hands.

Squeeze a crystal, get a voltage

Quartz is useful because it is piezoelectric. In 1880, Pierre Curie and his older brother Jacques found that putting pressure on certain crystals created electricity, as APS News recounts. The name comes from piezein, Greek for “squeeze”. Quartz showed one of the strongest effects.

The effect also runs backward. The next year, mathematician Gabriel Lippmann worked out that an electric field should make such a crystal deform. Your clock relies on that reverse effect. NIST’s time and frequency glossary puts it simply: apply a voltage and the crystal expands or contracts; reverse the voltage and the strain reverses. Drive it with a voltage that swings back and forth, and it vibrates.

A tuning fork in a tiny can

The quartz in a clock is cut into a miniature tuning fork and housed in a small can on the circuit board. Crystal maker Raltron, for example, sells 32.768 kHz tuning-fork crystals designed for time-of-day clocks, in a package about 3 by 8 millimeters.

Like a real tuning fork, it has a natural rate at which it rings, set by the cut and size of the crystal. The chip keeps it ringing. NIST describes how: the circuit takes the small voltage from the crystal, amplifies it and feeds it back, topping up each vibration with just enough energy. Quartz oscillators are the most common source of time and frequency signals, NIST says, and billions are made every year.

Counting to 32,768

Why such an odd number? Because 32,768 is 2 multiplied by itself 15 times. A digital circuit can halve a frequency easily: one simple stage puts out one pulse for every two it receives. Chain 15 of those stages, and 32,768 vibrations per second become exactly one pulse per second. That is the division by 32,768 that NIST describes.

A high frequency also keeps small errors small. NIST’s example is a quartz watch whose crystal is off by a tenth of a hertz. It would drift by one second every 327,680 seconds, or roughly 0.26 seconds a day.

The tick: a magnet that flips half a turn

The once-a-second pulse goes to a tiny stepping motor. A Seiko Instruments patent for an analog timepiece lays out the parts: a coil wound on a stator of magnetic material, and a two-pole rotor sitting in a hole in the stator. A Timex patent calls this family the Lavet-type stepping motor, with a bipolar permanent-magnet rotor, the kind that drives the hands through a gear train.

When a pulse flows through the coil, the stator becomes a magnet for a moment and the rotor turns 180 degrees. The next pulse has the opposite polarity, so the rotor turns another half turn in the same direction. That small, sudden step, passed through the gears to the second hand, is the tick you hear.

Some movements also make sure each step happened. The Seiko patent adds a check: it starts with a normal drive pulse and uses a wider, stronger one only when a detection circuit finds that the rotor did not turn.

Not every movement ticks. Clock-movement supplier ClockParts sells continuous-sweep movements, also run on one AA cell, whose second hand glides instead of stepping, so you don’t hear a click every second. At the other extreme, the Timex patent describes a watch motor that steps just once a minute.

From battery to second hand

1. Battery. A single AA cell powers the chip and the motor.
2. Crystal. The chip keeps the quartz tuning fork vibrating 32,768 times a second.
3. Divider. Fifteen halving stages count those vibrations down to one pulse per second.
4. Coil. The pulse briefly magnetizes the stator, with the polarity flipped each second.
5. Rotor. The little magnet turns half a turn, always in the same direction.
6. Gears. A train of small gears slows the rotor’s motion and turns the hands.

The gear ratios are simple arithmetic. The rotor makes half a turn each second, and the second hand goes round once in 60 seconds, so the gears between them slow things down 30 times. The minute hand turns 60 times slower than the second hand, and the hour hand 12 times slower again. Exact tooth counts vary from one movement to another.

How accurate it is, and why temperature matters

NIST lists temperature, humidity, pressure and vibration as changes that can shift a quartz resonator’s frequency, and slow aging of the crystal limits it over the long term.

Temperature is the big one for a tuning-fork crystal. Its frequency follows an upside-down parabola: fastest at a “turnover” temperature, which Raltron’s datasheet puts at 25°C, give or take 5 degrees, and slower whether it gets warmer or colder. With the datasheet’s typical curvature of 0.034 parts per million per degree squared, we calculate that a crystal 20 degrees from its turnover point runs about 14 parts per million slow, roughly 1.2 seconds a day. That is a calculation for the crystal alone, not a measured clock figure.

Each crystal also leaves the factory slightly off its nominal frequency: the standard tolerance for that part is plus or minus 20 parts per million. So a clock hung near a heater or on a cold outside wall can be expected to drift more than the same clock in a steady room.

Clocks that set themselves

Radio-controlled clocks contain a small radio. In the US, it listens to NIST station WWVB, which broadcasts on 60 kHz from near Fort Collins, Colorado. NIST says most decode the signal once a day and keep time with their own quartz oscillator in between. They are often sold as “atomic” clocks, which NIST calls inaccurate: an atomic clock has an atomic oscillator inside, such as cesium or rubidium. A plain battery clock has no radio and relies on its crystal alone.

From a lab bench to every wall

Warren Marrison joined Bell Laboratories in 1925 to work on frequency standards for radio transmission. In 1927, according to the National Inventors Hall of Fame, he divided a quartz crystal’s frequency electronically until it was low enough to drive a synchronous motor. Almost as a side effect, he had built the first quartz crystal clock. By the 1940s, time standard laboratories worldwide had switched from mechanical clocks to quartz.

The IEEE Milestone for the Seiko Quartz-Astron 35SQ, introduced in Tokyo on December 25, 1969, credits a Suwa Seikosha team led by Tsuneya Nakamura. It combined a quartz crystal oscillator, an integrated circuit and a miniature stepping motor to turn the hands, the same three jobs your wall clock does today. It was accurate to within five seconds a month.

Keeping a quartz clock accurate and running

• Try a fresh battery first. Howard Miller says old, exhausted batteries are the main cause of a battery movement misbehaving, and advises brand-new alkaline batteries, installed the right way round.
• Don’t mix batteries. Never mix old with new, or different types. Energizer warns this can cause leakage or rupture.
• Check the contacts and hands. A poor battery connection can cause trouble. If the clock runs slow, make sure the hands don’t rub the dial or glass.
• Set the time with the knob. Howard Miller’s setting guide recommends the hand-set knob. On non-chiming movements you may also push the hour hand forward, but never backward, which can strip the gears.
• Pick a steady spot. Away from heaters and cold walls, the crystal stays nearer the temperature where it keeps its best time.
• Take the battery out for storage. Energizer advises removing batteries from devices that won’t be used for a long time. Alkaline cells contain potassium hydroxide, which MedlinePlus lists as a poisonous ingredient, so a leak is worth avoiding.

If fresh batteries and free hands don’t bring it back, the movement is replaced as a unit. Howard Miller notes that the gears in battery-operated movements can’t be replaced or repaired.