# How do analog clocks/watches tick at exactly a second?

52 views

How do analog clocks/watches tick at exactly a second?

In: 6

Gears inside the watch have teeth spaced just right to allow a second to pass from one tick to the next.

Both use some physical process that takes a calculated/known period of time.
For a clock, it was how long a weight on a pole called a pendulum takes to swing.
The weight would swing from one side, then back. Gravity isn’t changing enough to matter so it will take the same time each swing.
When the pendulum was at the top, it would let a gear move one gear tooth (this is the tick sound). They’d also boost the pendulum up a bit, because otherwise it would stop swinging after a long time.
For watch, they have a spring coiling and uncoiling instead of a pendulum, but otherwise worked the same way.

They have some kind of [escapement gear](https://upload.wikimedia.org/wikipedia/commons/2/29/Anchor_escapement_animation_217x328px.gif). You can see from that gif that as the gear turns there is an arm (called the *pallet*) that catches the gear teeth so that the gear cannot continue rotating. A gear causes the pallet to oscillate back and forth and as it does so, it frees the gear to move *one* tooth before swinging back into place and stopping the gear.

A spring keeps constant tension on the gear so that it always wants to rotate, as soon as the pallet is out of the way. In [this gif](https://i.imgur.com/Wzdjaby.gif) you can see the spring twisting back and forth on the mechanism to oscillate the pallet. The pressure from the rotating escapement gear imparts energy back into the controller spring to keep it oscillating. That energy comes from the mainspring, which is the spring that you have to wind to keep the watch working (or else the watch is powered with a tiny electric motor). A wall clock might use a spring, an electric motor, or counterweights that are pulled by gravity. As the counterweights fall, they pull on a gear that powers the whole thing.

Instead of a controller spring, a clock may use a pendulum. A quirk of pendulums is that how far you lift them up doesn’t change how quickly they oscillate. Only the length and weight of the pendulum changes its period. Like the controller spring, as long as the period of the pendulum is known, then it can be used to control a pallet that controls an escapement gear which can be tuned with ratios to turn the hands at the right time.

The size and tension of the controller spring, along with the size and weight of the flywheel attached to it, control how quickly the pallet oscillates. Precise gear ratios connect the escapement gear to other gears which ultimately control the hands on the watch or clock. As long as the pallet is oscillating at a regular, predictable rate, the gears can be tuned with the ratios to turn at more or less exactly one second (and minute, and hour).

The precision of the size of the gears determines how accurate the clock or watch will be. And, as the energy source runs out – either the spring needs to be wound, the counterweights lifted, or the battery replaced, it doesn’t add as much energy back into the controller spring and it will inevitably slow down, making the mechanism less accurate.

A pendulum clock with a long enough pendulum (0.994 m = 39.1 in) will try to swing at exactly one second intervals, regardless of the weight. Until friction saps its energy and slows it down, that is. It’s one of the cool things about pendulums.

To keep supplying energy to the pendulum so that friction doesn’t make it slow down and stop, they use a special gizmo called an [escapement mechanism] (https://en.wikipedia.org/wiki/Escapement). This can also be used with a shorter pendulum to keep it running at 1 second periods.

An analog watch that uses springs to supply the energy also uses an escapement mechanism to regulate how fast the gears move as the mainspring slowly uncoils.

Believe it or not it has to do with the function of a simple pendulum. A heavy weight that is a certain length away from a pivot point. We can determine the rate at which a pendulum will swing back and forth simply through knowing the distance the weight is from the pivot and the force applied to the pendulum to get it swinging. So you use a little math to make a pendulum that will swing with a period of 1 or 2 seconds exactly.

Now we make a device to drive that pendulum. In turn the resistance from that pendulum will regulate the rate at which the device advances. If we measure the number of swings of that pendulum (divided by 2 if it’s a 2 second interval) and then display that output in increments of 60 on a dial, then you have a clock.

Wristwatches take that pendulum concept and turn it on its side. The force of tension from the length holding the weight to the pivot is what forces the weight to swing back the other way. Thus we just need something that can hold tension. So we place a spring inside a wheel of a specific size and weight. In the center of that wheel we place a thin pin (to minimize friction). We send that wheel spinning. As it sounds one way it build tension on the spring until the tension overcomes the force of rotation and sends it back spinning the opposite direction. It changes direction of spin every second. Now we scale down our mechanism we used in the grandfather clock and use this new spinning pendulum to keep the mechanism at a steady 1 second pace.

Now it’s not perfect due to inefficiencies in the system most wristwatches and grandfather clocks can lose a second or two each day. Also we have to keep the spring driving the whole mechanism wound like a wind up toy. However for a first accurate time piece it works well.

Real mechanical clocks use a wound spring (usually, could be other forms of stored energy) to store energy, slowly releasing it by turning gears that have precise numbers of teeth based on the ratios between seconds, minutes, and hours. The “ticks” are regulated by the oscillation of a specific mass, like a swinging pendulum, or wheel that repeatedly turns back and forth. The mass is “tuned” based on the mass/energy of the whole system to switch directions at precise intervals. A big pendulum clock might “tick” once per second or more, while a watch can “tick” several times per second, with the gearing adjusted accordingly.

I just saw a great write up with interactive animations that broke it all down really well. It was posted on Reddit somewhere, but I forget where. Hopefully someone posts it here for you.

Every clock has some timekeeping element, the purpose of which is to move at consistent intervals.

Common time keeping elements include:

1. piezoelectric quartz crystals which vibrate at defined intervals in a specific electrical circuit (more details here https://en.wikipedia.org/wiki/Crystal_oscillator?wprov=sfla1)

2. torsion and pendulum escapement mechanisms – basically a gear and pendulum (escapement), which ensures the gear takes some defined time to turn when a constant turning force (torsion) is applied to it by a wind-up spring. This works because pendulums subject to constant forces, have a constant frequency with which they go back and forth.

Here’s a link with a good animation:

Now after you have any turning motion at a defined frequency, converting it to seconds is easy using gears. E.g. If you have a gear with 30 teeth rotating every 30 seconds, you just need another gear with 60 teeth connected to it, to get your rotation every 60 seconds.

Here, you’re going to want to read and play with the interactive things on this amazing site: [https://ciechanow.ski/mechanical-watch/](https://ciechanow.ski/mechanical-watch/)

But the answer is that there is a spring, called the balance spring. It spins back and forth. They pick a material and a spring tightness that is going to spring back and forth at a pretty predictable rate (perhaps 4 times per second). They build it with little adjustable weights that they can push back and forth a bit to slightly speed up or slow down the spring, and they test and adjust it before it leaves the manufacturer so that it’s as close to perfect as possible.

The spring’s attached to a balance wheel, which hits a pallet fork repeatedly, which causes a gear to tick forwards some nearly exact number of times per second.

Watches that tick once a second are usually powered by electronic movements. Mechanical escapements in a watch are far too tiny to get a pulse a second out of.

The electronic circuit has a quartz crystal (hence the early term, “quartz watch.”) The crystal is cut to vibrate at exactly 32,768 times a second when stimulated by a tiny electric voltage. Crystals are fairly accurate over a reasonable range of temperature, so they keep the time quite well.

The oscillations then go through a series of frequency divider circuits that successively divide them by powers of two (32768 – 16384 – 8192 – 4096 – 1024 – 512 – 256 – 16 – 8 – 4 – 2 -1.)

Once the output circuit is providing one pulse per second, the signal is amplified and used to rotate a tiny permanent magnet using a coil of wire (electromagnet). The magnet is geared to the watch hands and voila! You get a nice analog display with the second hand ticking once a second.

When the pivot of a wheel that is free to spin is secured to the centre of a coiled leaf spring, the wheel will oscillate back and forth at a predictable, and very steady rate.

Special mechanisms called escapements restrain a spring driven clock movement, advancing it incrementally with each oscillation of the wheel (called a balance wheel). During each oscillation, the escapememt transfers a little energy from the movement it restrains to the balance wheel, which sustains the balance wheel’s operation.