How do clocks work?
From a grandfather clock to the atomic clocks behind GPS, every clock does the same three jobs. Here is how each kind does them.
Three jobs every clock does
Every clock, however old or new, needs three things: a source of power, something that repeats at a steady rate, and a way to count those repeats and show the result.
In a grandfather clock the power is a falling weight, the steady beat is a swinging pendulum and the counting is done by gears that turn the hands. In a battery clock the power is the battery, the beat is a vibrating quartz crystal and the counting is done by a tiny circuit. The details change; the three jobs never do.
Mechanical clocks: power and gears
A mechanical clock is driven by a weight hanging on a cord or chain, or by a coiled spring that slowly unwinds. Left alone, either would run down in seconds and spin the gears in a blur.
The power passes through a chain of gears called the train. Each gear turns the next at a carefully chosen speed, so that one wheel turns once an hour to carry the minute hand and another turns once every twelve hours for the hour hand. Winding the clock simply lifts the weight or tightens the spring again.
See also: a labeled diagram of the parts inside a clock on OnlineClock.app.
The escapement: the part that ticks
What stops a clock from running down all at once is the escapement. It locks the gear train, lets it advance by one tooth, then locks it again, over and over. The tick you hear is a tooth landing on the escapement.
Each release is timed by something that swings back and forth: a pendulum in most wall and floor clocks, or a balance wheel and spring in carriage clocks and mechanical watches. As each tooth slips past, it also gives the swinging part a small push, so it never runs out of energy. Britannica says the first escapement, the verge, was probably invented in 13th-century Europe.
Why the pendulum keeps good time
A pendulum of a given length always takes about the same time to swing, which NIST describes as a natural period of oscillation. That steady rhythm comes mostly from its length. A pendulum about 39 inches long takes about a second for each swing, which is why grandfather clocks are so tall.
Christiaan Huygens built the first working pendulum clock in 1656. NIST says it erred by less than a minute a day, far better than earlier clocks, and that by 1721 George Graham had brought pendulum clocks to about one second a day by correcting for the way heat and cold change a pendulum's length.
Quartz clocks: a vibrating crystal
Most clocks and watches today run on quartz. When electricity is applied to a quartz crystal, it vibrates at a constant frequency, a property called piezoelectricity. In most clocks and watches the crystal vibrates 32,768 times a second.
That number is 2 to the 15th power. The crystal maker Jauch explains that the circuit halves the frequency 15 times, leaving exactly one pulse per second. Each pulse steps a tiny motor that moves the hands, or updates a digital display. NIST's Michael Lombardi notes that quartz oscillators can keep time to about 100 microseconds a day, though temperature and vibration affect them.
Some battery clocks have a continuous sweep movement instead. Its second hand glides smoothly rather than jumping once a second, which, as a clock parts supplier explains, also means no click every second. These are the clocks sold as silent or non-ticking.
Atomic clocks and the atomic clock on your wall
Atomic clocks take their beat from atoms, which always absorb and give off energy at exactly the same frequencies. Since 1967 the official second has been defined by 9,192,631,770 cycles of a frequency of cesium atoms, and NIST notes that the best modern clocks use light rather than microwaves to tick even faster.
Clocks sold as atomic for the home contain no atoms. They are quartz clocks with a small radio receiver that picks up a time signal from an atomic clock. In the United States that signal is NIST's station WWVB near Fort Collins, Colorado. Most of these clocks listen once a night and keep time with their quartz crystal in between.
See also: a live analog clock to watch the hands move on OnlineClock.app.
Questions people ask
How does a clock work in simple terms?
It uses a source of power to keep something repeating at a steady rate, such as a pendulum or a quartz crystal, then counts the repeats with gears or a circuit and shows the result on the dial.
What makes a clock tick?
In a mechanical clock, the tick is a gear tooth landing on the escapement. In a quartz clock, it is the small motor stepping the second hand forward once a second.
How does a battery clock keep time?
A quartz crystal inside vibrates 32,768 times a second. A circuit counts the vibrations and sends one pulse a second to the motor that moves the hands.
Do atomic wall clocks have atoms in them?
No. They are quartz clocks that set themselves by a radio time signal broadcast from a real atomic clock.
Keep exploring
Sources
- Escapement, Encyclopaedia Britannica
- A Walk Through Time: A Revolution in Timekeeping, NIST
- Time Measurement (Lombardi, NIST, 1999)
- Why do tuning fork crystals have a frequency of 32.768 kHz?, Jauch
- Continuous sweep clock movements, Clockparts.com
- How do atomic clocks work?, NIST
- Help with WWVB radio controlled clocks, NIST
- Optical clocks: the future of time, NIST
- Longcase clock with calendar, Thomas Tompion, about 1700, The Metropolitan Museum of Art
More clock guides
How to identify an antique clock and judge its value · How to choose a good clock · How to care for any clock · Clock hands and numbers · Clock movements explained · Grandfather clock weights, winding and care · Why clocks use IIII instead of IV · Tempus fugit: the motto on the clock · How to date a watch by its serial number · The Westminster chime · Who invented the clock? · all guides