Quartz clocks and watches
by Chris Woodford. Last updated: September 19, 2012.
You may not believe in astrology, but there's no question the planets rule our lives. We get up when the Sun rises (or some time after) and go to bed when it sets. We have a calendar based on days, months, and years—periods of time that relate to how the Moon and Earth move around the Sun in the sky. For most of history, people found this kind of "astronomical timekeeping" good enough for their needs. But as the world became ever more frantic and sophisticated, people needed to keep track of hours, minutes, and seconds as well as days, months, and years. That meant we needed accurate ways of keeping time. Pendulum clocks and mechanical watches used to be the best way of doing this. Today, many people use quartz clocks and watches instead—but what are they and how do they work?
Photo: Quartz is really cheap and the clocks that use it need hardly any moving parts. That's why it's now used in even the most inexpensive timepieces. Because it's so accurate and reliable, it's very much a selling point—which is why clocks like this proudly have the word "quartz" plastered prominently across their dials.
How ordinary clocks work
We all know that a clock keeps time, but have you ever stopped to think about how it does so? Probably the simplest clock you could make is a speaking clock. If you count seconds by repeating a phrase that takes exactly one second to say (Like "elephant one", "elephant two", "elephant three"...), you'll find you can keep time pretty accurately. Try it out. Say your elephants from one to sixty and see how well you keep time over a minute, compared to your watch.
Not bad, eh? The trouble is, most of us have better things to do all day than say "elephant." That's why people invented clocks. Some of the earliest clocks used swinging pendulums to keep time. A pendulum is a long rod or a weight on a string that swings back and forth. In 1583, the Italian physicist Galileo Galilei (1564–1642) discovered that a pendulum of a certain length always takes the same time to swing back and forth, no matter how heavy it is or how big a swing it makes. He figured this out by watching a huge lamp swinging on a chain from the ceiling of Pisa Cathedral in Italy, and using his pulse to time it as it moved back and forth. In a clock, the pendulum's job is to regulate the speed of the gears (interlocking wheels with teeth cut into their edges). The gears count the number of seconds that pass and convert them into minutes and hours, displayed on the hands that sweep round the clockface. To put it another way: the gears in a pendulum clock are really just counting elephants.
Photo: Pendulum power: This swinging rod (with a weight at the bottom) is what keeps the time in a grandfather clock. It was one of the great discoveries we owe to Galileo.
You can make a pendulum clock by tying a weight to a piece of string. If the string is about 25cm (10 inches) long, the pendulum will swing back and forth roughly once each second. Shorter strings will swing faster and longer strings slower. The trouble with a clock like this is that the pendulum will keep stopping. Air resistance and friction will soon use up its energy and bring it to a halt. That's why pendulum clocks have springs in them. Once a day or so, you wind up a spring inside the clock to store up potential energy to keep the pendulum moving for the next 24 hours. As the spring uncoils, it powers the gears inside the clock. Through a see-saw mechanism called an escapement, the pendulum forces the gears to turn at a precise rate—and this is how the gears keep time. A pocket watch is obviously too small to have a pendulum inside it, so it uses a different mechanism. Instead of a pendulum, it has a balance wheel that turns first one way and then the other, controlled by a much smaller escapement than the one in a pendulum clock.
How quartz clocks work
Photo: Crystals of quartz. Photo by courtesy of US Geological Survey.
The trouble with pendulum clocks and ordinary watches is that you have to keep remembering to wind them. If you forget, they stop—and you have no idea what time it is. Another difficulty with pendulum clocks is that they depend on the force of gravity, which varies very slightly from place to place; that means a pendulum clock tells time differently at high altitudes from at sea level! Pendulums also change length as the temperature changes, expanding slightly on warm days and contracting on cold days, which makes them less accurate again.
Quartz watches solve all these problems. They are battery powered and, because they use so little electricity, the battery can often last several years before you need to replace it. They are also much more accurate than pendulum clocks. Quartz watches work in a very different way to pendulum clocks and ordinary watches. They still have gears inside them to count the seconds, minutes, and hours and sweep the hands around the clockface. But the gears are regulated by a tiny crystal of quartz instead of a swinging pendulum or a moving balance wheel. Gravity doesn't figure in the workings at all so a quartz clock tells the time just as well when you're climbing Mount Everest as it does when you're at sea.
Quartz sounds exotic—with a "q" and a "z," it's a great word to play in Scrabble—but it's actually one of the most common minerals on Earth. It's made from a chemical compound called silicon dioxide (silicon is also the stuff from which computer chips are made), and you can find it in sand and most types of rock. Perhaps the most interesting thing about quartz is that it's piezoelectric. That means if you squeeze a quartz crystal, it generates a tiny electric current. The opposite is also true: if you pass electricity through quartz, it vibrates at a precise frequency (it shakes an exact number of times each second).
Photo: The quartz oscillator from a watch. You can see how small it is by looking at the very last photo on this page. This is the part numbered "5" in that picture.
Inside a quartz clock or watch, the battery sends electricity to the quartz crystal through an electronic circuit. The quartz crystal oscillates (vibrates back and forth) at a precise frequency: exactly 32768 times each second. The circuit counts the number of vibrations and uses them to generate regular electric pulses, one per second. These pulses can either power an LCD display (showing the time numerically) or they can drive a small electric motor (a tiny stepping motor, in fact), turning gear wheels that spin the clock's second, minute, and hour hands.