Anatomy of a Second

People have always measured time with things that repeat: sunrise, the phases of the moon, a heartbeat. In ancient India, astronomers used a water clock called a ghaṭī. It was a small copper bowl with a hole in the bottom. Placed in a basin of water, it slowly filled and sank. One sinking took one ghaṭikā, or 24 minutes. Someone then emptied the bowl, set it floating again, and added one to the count.

That bowl has the same four basic parts as any clock, including the watch on your wrist.

Of the four, the oscillator matters most. Gears count perfectly and hands point perfectly, but if the oscillator speeds up or slows down, the clock will be wrong. Most of the history of clocks is about finding steadier oscillators.

The first really good one was the pendulum. According to a well-known story, a young Galileo was sitting in Pisa Cathedral when he noticed a hanging lamp swinging. He timed it against his pulse. Big swings and small swings seemed to take the same time. Only the length of the chain made a difference.

Try it below. Change the length of the string and watch how the time for each swing changes.

A pendulum only works when it hangs still and gravity pulls straight down. On your wrist, every step would throw it off. A watch needs an oscillator that works in any position. The next four chapters show how a mechanical watch solves this.

A mechanical watch has between 100 and 300 parts, and many of them are smaller than a grain of rice. It runs without any electricity.

Below is an automatic watch, the kind that winds itself when you move your arm. As you scroll, it comes apart one layer at a time, from the glass on top to the steel back. Each part is explained as it lifts out.

The clear cover over the dial is called the crystal. The name goes back to the 1600s, when some watch covers were cut from rock crystal, a clear form of quartz. Today it has nothing to do with the quartz crystal that keeps time inside a quartz watch. The two just share a name.

A crystal has a simple job: let you see the time, and keep out dust, water and fingers. But it gets knocked, rubbed and scraped every day, so the material matters a lot. Modern watches use one of three.

Why hardness matters

Ordinary dust and sand are mostly tiny grains of quartz, which has a hardness of 7. Anything softer than 7, like plastic or glass, gets slowly scratched just by daily life: wiping the watch on a sleeve, brushing against a wall, a day at the beach. Sapphire, at 9, is harder than sand, so those scratches don't happen. Very few everyday things, diamond among them, are hard enough to scratch it.

How sapphire is made

In 1902, the French chemist Auguste Verneuil found a way to grow sapphire. Fine aluminium oxide powder is dropped through a very hot flame. It melts and builds up, drop by drop, into a single crystal. The same method made synthetic rubies cheap, which is why every mechanical watch could have jewel bearings.

Today, sapphire for watches is grown in large furnaces, then sliced and ground into shape. It has to be cut and polished with diamond tools, because almost nothing else is hard enough. Sapphire crystals became common on expensive watches in the 1970s and 1980s, and are now found on many mid-priced watches too.

Because sapphire reflects more light than glass, many sapphire crystals have a very thin anti-reflective coating, the same idea as the coating on spectacle lenses. It makes the crystal almost disappear when you look straight at the dial.

A mechanical watch is powered by a single strip of springy metal, 20 to 30 cm long, coiled up inside a small drum called the barrel. When you turn the crown, you wind the spring tighter around the post in the middle of the barrel. As the spring slowly unwinds, it turns the barrel, and the teeth on the barrel's edge turn the rest of the gears.

A fully wound watch usually runs for about 40 hours. This is called the power reserve. Some newer watches run for 70 or 80 hours, so a watch you take off on Friday evening is still running on Monday morning.

That uneven push is a problem. If the gears simply turned as fast as the spring pushed them, the watch would run fast in the morning, just after winding, and slow by the evening.

So the spring doesn't control the speed. It only supplies the power. A different part, the escapement (Chapter V), controls how quickly that power is used. Between the two is a chain of gears, which comes next.

Gears are good at counting. If a wheel with 60 teeth turns a smaller gear with 8 teeth, the small gear has to turn 7½ times for every turn of the big one. It can't do anything else.

In watches, the small gears are called pinions and their teeth are called leaves. A watch links four or five wheel-and-pinion pairs into a chain called the gear train. The barrel at the start of the chain turns about once every 6 to 8 hours. The escape wheel at the end turns 16 times a minute.

The numbers of teeth are chosen so that one wheel turns exactly once an hour and another turns exactly once a minute. The minute hand is attached to the first, and the seconds hand to the second.

The hour hand has to turn once every 12 hours, not once an hour. A small set of gears under the dial, called the motion works, slows the minute hand's motion down by 12 to 1 to drive the hour hand.

If you removed the escapement from a watch, the spring would unwind in a few seconds and the hands would spin until it ran out. Watchmakers sometimes do this on purpose, carefully, to release the spring before taking a watch apart.

The escapement works like a brake that releases the gears in tiny, even steps. Its timing comes from the balance wheel, a small wheel that swings back and forth on a coiled hairspring. It does the same job as a pendulum, but it works in any position.

Each time the balance swings through the middle, it unlocks the escape wheel, which moves forward by half a tooth. As it moves, the escape wheel gives the balance a small push that keeps it swinging. That push, and the click that follows it, is the tick you hear.

So the balance sets the pace, and the spring supplies the energy.

Why the balance keeps good time

A balance wheel takes almost the same time for every swing, whether the swing is big or small, just like Galileo's lamp. When the spring is fully wound, the balance swings further. When the spring is nearly empty, it swings less far. Either way, each swing takes about the same time. That's why a weaker spring doesn't make the watch run slower.

It isn't perfect. Gravity affects the balance differently depending on how the watch is lying. Heat changes how springy the hairspring is. Magnets can affect it too. Even a good mechanical watch gains or loses a few seconds a day.

Why watches use rubies

Every axle in a watch turns in a small hole. Steel turning in brass would wear the hole out within a few years, so the most important holes are lined with synthetic ruby. Ruby is very hard and very smooth, and it holds a tiny drop of oil in place. Ruby is the same mineral as the sapphire used for watch crystals. A basic wind-up watch has 17 of these jewels. Synthetic rubies have been cheap since the early 1900s, so they're there to reduce wear, not to add value.

On 25 December 1969, Seiko started selling the Astron in Tokyo. It was the first quartz wristwatch on sale anywhere. It was accurate to within 5 seconds a month, while a good mechanical watch of the time could be off by that much in a single day. It also cost about as much as a small car.

Prices fell quickly. Like other electronics in the 1970s, quartz watch parts got smaller and cheaper every year. By the mid-1980s, a quartz watch more accurate than any mechanical watch cost very little. Switzerland's watch industry lost a large share of its jobs. This period is known as the quartz crisis.

Scroll down to take a quartz watch apart. Compare its layers with the mechanical watch above.

Quartz is a common mineral made of silicon and oxygen. It has an unusual property. Squeeze it, and it produces a small voltage. Put a voltage across it, and it bends slightly. Pierre and Jacques Curie discovered this in 1880. It's called piezoelectricity.

In a watch, the quartz is cut into a tiny tuning fork about the size of a grain of rice. A circuit gives it a small electrical push in time with its own movement, so it keeps vibrating, much like a bell that keeps ringing. Each fork vibrates at one very precise rate, set by its size and shape.

The quartz crystals in watches are made to vibrate exactly 32,768 times a second. (This is a different thing from the crystal on top of the watch, which is just the clear cover.) That number is 2 multiplied by itself 15 times, or 2¹⁵.

Why this number? First, because 32,768 is a power of 2, halving it 15 times gives exactly 1 pulse a second. Second, it's above the highest pitch people can hear (about 20,000 vibrations a second), so the watch doesn't make a whining sound. Third, a fork of this size is small and uses very little power. A slower crystal would have to be bigger, and a faster one would drain the battery sooner.

The same kind of crystal is inside almost every phone, laptop and microwave oven. It keeps their clocks running even when they're switched off.

Most quartz watches with hands use a silver-oxide button cell about the size of a shirt button. Inside are two materials, zinc and silver oxide. The zinc gives up electrons and the silver oxide takes them in. A thin layer inside the battery, called the separator, stops the electrons going straight across. Instead they travel out of the battery, through the watch's circuit and motor coil, and back in. That flow of electrons is the electric current that runs the watch.

Zn + Ag₂O → ZnO + 2 Ag  ·  about 1.55 volts

Watches use silver-oxide cells because their voltage stays almost the same until they're nearly empty, which suits a timekeeping circuit. As the battery is used up, the zinc turns into zinc oxide and the silver oxide turns into silver metal.

Other ways to power a watch

Light. Solar watches have a thin solar cell under the dial, which lets some light through. The cell charges a rechargeable battery. Citizen sold one of the first in 1976. A good one can keep running for months in a drawer.

Movement. An automatic mechanical watch uses its swinging rotor to wind the mainspring. Some quartz watches use the same kind of rotor to turn a tiny generator that charges a battery-like part called a capacitor. Seiko introduced this in 1988 and later named it Kinetic.

Your hand. The oldest method is winding the crown by hand, usually once a day.

Each big improvement in timekeeping came from a better oscillator. The pendulum replaced the rough mechanisms of medieval tower clocks. The balance wheel and hairspring made accurate portable watches possible. Quartz replaced the balance wheel. Then atoms replaced quartz. Since 1967, the second has been officially defined using the caesium atom: one second is exactly 9,192,631,770 vibrations of a caesium-133 atom.

The chart shows how much each kind of clock typically drifts in a day. Each gridline is 1,000 times smaller than the one to its right. On a normal scale, everything after the pendulum would be too small to see.

This timeline runs from ancient Egypt to today, with stops in China, Europe, India and Japan. Dates before about 1600 are approximate, and historians disagree about some of them.

By the 1980s, quartz watches were cheaper and more accurate than mechanical ones. Even an inexpensive quartz watch keeps better time than a handmade Swiss one. Mechanical watches came back anyway. Many people like them for the craftsmanship, in the same way some people still prefer handwritten letters. Quartz kept improving too. Today some watches set themselves from atomic clocks, and some run for years on light.