Mechanical
watch.
How it works Inside the case
The numbered face is the dial; the pointers are the hands. The clear cover above them is the crystal.
The small knob on the side is the crown. Under the dial sits the movement: the mechanism that powers and moves the hands.
Everything that follows happens inside the movement. It has three jobs: store energy, let it out at a steady pace, and count the result on the hands.
What powers it?
Turning the crown coils a ribbon of steel more tightly. This ribbon is the mainspring. It stores the energy you put in by winding.
The spring sits inside a round container called the barrel. As the spring tries to uncurl, it pushes the barrel around. That gives us a source of turning motion.
Move the slider to see the coils gather around the centre as the spring is wound. The barrel is held still in this view.
A full wind turns the barrel about seven times, enough to run the watch for a little over two days. But a barrel that turns seven times in two days is no use to a hand that must turn once a minute. The motion has to travel, and speed up.
How does the motion travel?
Teeth around the barrel’s edge push a much smaller gear, called a pinion, on the next axle. The pinion has far fewer teeth, so it turns faster than the barrel, and in the opposite direction.
The pinion shares its axle with a larger wheel. They turn together, and that wheel drives the next pinion. This chain of wheels and pinions is the gear train. Each wheel turns faster than the one before it.
Three wheels have names worth knowing. The centre wheel sits in the middle of the movement and turns once an hour. Two wheels on, the fourth wheel turns once a minute. Last and fastest is the small purple escape wheel, at sixteen turns a minute.
The barrel turns once every eight hours. Even at the 6× speed shown here, one turn takes 80 minutes, so the barrel and centre wheel barely seem to move. Watch the fourth wheel and the purple escape wheel instead.
The train turns one slow, powerful push into many fast turns. But nothing tells the barrel how quickly it may unwind.
Why wouldn’t that keep time?
So far we have a spring, a barrel and a chain of gears, with nothing to hold them back. Watch what the barrel does on its own.
This experiment starts with a wound spring and the barrel at rest.
It surges, then slows as the spring weakens and friction takes its toll. A real movement left like this would spin down in seconds. A hand driven by it would move, but it would not mark equal seconds.
A clock needs two things the train lacks: a motion that repeats at a steady pace, and a way of making the gears wait for it.
What can set the rhythm?
The balance wheel and its fine hairspring provide the steady, repeating motion. Left to itself, this balance swings back and forth four times every second.
Watch the red mark on the rim. The hairspring pulls the wheel towards its resting position. The wheel’s momentum carries it past that position, so the spring pulls it back again. One full back-and-forth swing is an oscillation.
Without another push, each swing gets smaller until the wheel stops. This experiment exaggerates that energy loss so you can see it happen.
What sets the pace? A stiffer spring pulls harder and makes the wheel swing faster. More inertia—from a heavier wheel or weight farther from its centre—makes it harder to reverse, so it swings more slowly.
Change either slider to release the wheel again. Compare its pace and the number below. These sliders affect this experiment only.
at actual speed
The pace is set by the spring and the wheel alone, not by how hard it was pushed. That is what makes it a good clock. The next step is to let this rhythm control the gears—and to use a little of the mainspring’s energy to keep the balance swinging.
How does the rhythm control the gears?
The purple escape wheel is the last wheel of the gear train. The faint wheel above it is the fourth wheel that drives it. Beside it sits the yellow fork, which rocks on its own pivot. Together they form the escapement.
One of the fork’s two ruby stops rests against a tooth of the escape wheel and holds it. While that tooth is held, every wheel behind it is held too, right back to the barrel. The mainspring pushes, but nothing moves.
The balance decides when the fork lets go. Watch the circled red pin near the balance’s centre: once per swing it knocks the fork across. That frees the wheel for a moment, the other stop catches it, and in passing the fork gives the pin a small push. That push is what keeps the balance swinging.
The balance and hairspring are faded so the pin and fork stand out. The release is over in a blink even in slow motion, so drag the slider through it, or play one beat with the release stretched out.
This is the seconds hand, riding on the fourth wheel. Every release turns it ¾ of a degree: 480 equal steps make one turn, and eight of them pass each second.
Each release is a beat. It lets the escape wheel move exactly half a tooth, so every wheel behind it, and every hand, moves one small, fixed step. The gears can only advance in equal steps, and the balance decides when each step happens. That is what keeping time means.
How do the hands count time?
The dial is drawn see-through here, so you can see what each hand sits on.
The minute hand sits on a short hollow tube. The tube grips the centre wheel’s shaft, so wheel, tube and hand turn together: once an hour.
The hour hand sits on a wider purple tube around the minute tube, free to turn on its own.
Two small gears under the dial drive it. The first slows the motion three times, the second four times more. Three times four is twelve, so the hour hand makes one turn for every twelve of the minute hand.
The seconds hand sits straight on the fourth wheel’s shaft, which turns once a minute. That wheel sits low in the movement, so the hand gets its own small dial near the 6.
How does it all work together?
Energy travels from the mainspring through the wheels to the escapement. The fork holds it back. The balance lets it through one step at a time, and receives a little push each time it does.
The metal supports across the wheels are called bridges. They hold the axles in place. Hard jewel bearings let the axles turn with little friction and wear.
Highlight a part
PowerThe mainspring, wound through the crown, pushes the barrel round once every eight hours.
GearsWheels and pinions turn that slow push into faster turns. Two of them carry the minute and seconds hands.
Balance & forkThe fork holds the train still. The balance, swinging four times a second, frees it eight times a second, half a tooth at a time.