#1497 How does a watch keep time?

How does a watch keep time? Modern watches use a quartz movement to keep time, but mechanical watches use a wound spring.

Modern watches are probably easier to understand, so let’s look at those first. They work on the principle of piezoelectricity, which is a property that the mineral quartz possesses. The piezoelectric effect is the ability to produce an electric charge when mechanically stressed. That means, if you put a force on a piece of quartz, it will produce electricity. Conversely, if you put an electric charge into a piece of quartz, it will move. The reason we can use a quartz crystal to tell the time is because they move at a precise rate. They expand and contract 32,768 times a second. To tell the time, all you need to do is have a battery that supplies an electric charge to the quartz crystal and a microchip to count the number of oscillations. The electronic circuit from the battery keeps the quartz vibrating, and the microchip counts 32,768 oscillations and moves the time count on one second. Quartz watches are very

and, so long as they have battery power, they work without any input from the owner. They do lose a little bit of time depending on the quality of the watch. They can lose about 15 seconds a month, but a lot of modern watches reset themselves when they receive a radio signal, so the owner would probably never notice.

Mechanical watches are not as accurate, but they are probably more impressive because of their ingenuity. A mechanical watch works on a clockwork system. The owner winds the watch by twisting a dial, which winds a spring. The energy to power the watch comes from the user’s body in the winding process, and it is stored in the spring. A mechanical watch needs a winding mechanism, a spring, gears, a balance wheel, and something called an escapement mechanism. Let’s go through the mechanism in sequence.

The first step is the mainspring, which is where the energy is stored. It is a coiled piece of metal, usually steel, held in a round container. One end of the spring is connected to an axle called an arbor, and the other end is connected to the barrel. When you wind the watch, you twist the arbor, and the mechanical energy from your body is used to twist the spring tighter and the spring stores this energy. The spring wants to release all of this energy and return to its natural uncoiled state, but the barrel around it prevents this, so the spring holds the energy. The barrel has gear teeth around the edge of it that are connected to a series of gears called a gear train. As the barrel turns, the movement is passed along the train, turning all of the other gears. They are sized so that one gear rotates completely once an hour (the minute hand), one rotates completely once every twelve hours (the hour hand), and one rotates completely once a minute (the second hand). This is how the hands turn, but there is another vital part of the mechanism and this is called the escapement mechanism.

The problem with the watch mechanism is that there is no natural way to regulate how much energy the mainspring puts out. If you simply let the spring unwind, it would release all of its energy at once, and the hands would whiz around until all of the energy was gone. To prevent this, watches have a balance wheel and an escapement mechanism, which is an invention of pure genius. The balance wheel is a small, weighted wheel with a very thin spring attached to it. Left by itself, it rocks backwards and forwards at a steady rate, just like a tiny, fast pendulum. The escapement has a cog called the escape wheel and a pallet fork attached to it. The fork has a pivot in the middle and two small “legs” at the bottom. These legs alternately lock and unlock the teeth of the escape wheel. Power from the mainspring turns the gear train and tries to spin the escape wheel. One leg of the fork rests against a tooth and stops it from turning. At the top end of the fork, there is a notch that fits onto a pin on the balance wheel. As the balance wheel swings, the pin pushes the fork to one side. This briefly frees the escape wheel so it can move on by one tooth and, at the same time, gives a tiny push to keep the balance wheel swinging. The fork then pivots back the other way, stopping the escape wheel again with its other leg.

The balance wheel and spring keep swinging backwards and forwards, and each swing allows the escape wheel to move on by just one tooth. This means the gear train moves forwards in very small, regular steps instead of spinning freely. Those steps are the familiar “tick-tock” you hear from a mechanical watch. The escapement acts as a brake on the power coming from the mainspring and ensures that everything moves at a steady rate. A very complicated and incredibly well-designed machine. And this is what I learned today.

Sources

https://www.watches-of-switzerland.co.uk/calibre/inspiration/a-guide-to-mechanical-watch-movements

https://en.wikipedia.org/wiki/Mechanical_watch

https://en.wikipedia.org/wiki/Mainspring

https://en.wikipedia.org/wiki/Barrel_(horology)

https://www.nixon.com/blogs/stories/how-a-watch-works?srsltid=AfmBOoqwozPqOkAYY08nwPnt_2qtkr0JNrPDaZ_zk1w1mKrzEyduXTgd

https://www.nixon.com/blogs/stories/how-quartz-watches-work

https://en.wikipedia.org/wiki/Quartz