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Quartz Watch

Bill takes apart a cheap watch to show how it works. He describes how a tiny quartz tuning fork keeps the time.

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Transcript In some sense this everyday watch isn't special at all - I mean it for about ten bucks at Target yet once we look inside it's amazing. Let me show you.

I'd call it the key machine of the modern industrial age. Precise time keeping made possible things like the Global Positioning System and our tele-communications infra-structure.

Now, this metal container holds the heart of this watch. Inside lies a tiny quartz fork.

I have one: Now its so tiny that I keep it in this white cap. You can see it right there in the center. Now, although this modern circuity is vital to the watch, it's based on the same principles of the first clocks built in the 17th century: Resonate motion. It's easiest seen in the pendulum clock.

This family heirloom - it hung in the living room when I was a kid - uses the motion of a pendulum to keep time. Now, this pendulum oscillates with a regular period that runs a clockwork that translate this motion into movement of the hands. There are many ways to create resonate motion. For example, a tuning fork.

This one vibrates four hundred and forty times a second – that’s an A note when struck . . . I love that sound. Now, if I slow down the motion of the fork you can see how the tines move back and forth with a regular period. That's resonate motion, like the pendulum, which can be used to measure time.

The quartz crystal I showed from inside this watch is a tiny tuning fork. It vibrates at about 30,000 times per second. But how do you get it to vibrate and how do you measure or record its vibrations? I mean we cannot get a hammer in there to hit the quartz crystal.

The engineers who designed this digital watch used something known as the piezoelectric effect to make this small tuning fork vibrate. You can see this piezoelectric effect most easily with Rochelle Salt.

Here at the center lies the crystal. I've attached two electrodes made of tin foil, and strung wire from them to a small bulb. Watch what happens as I strike the crystal with a hammer. As I deform the crystal it generates a current. The reverse also happens: If you place a voltage across the crystal it deforms. This is how the quartz tuning fork in the watch is pinged: A voltage from the battery sets it in motion, and then the watch's circuitry measures the current fluctuations that represent the resonate motions of the tines. Quartz is ideal for digital watches because of its outstanding physical hardness, and mechanical and chemical stability; and that stability makes this watch work nearly anywhere on earth under all but the most extreme conditions. One more interesting thing about these digital watches is how these tiny tuning forks are made.

On a production line engineers must make these quartz tuning forks so accurate that they vibrate at 32,768 times a second plus or minus one six-hundredth - about two parts per million in frequency. If that frequency differs by one six-hundredth the watch will be off by more than one minute a year.

To see how they tune these forks look at the ends: You see deposits of gold at the ends of the tines. These are added to make the fork's vibration frequency too low. On the production line a laser zaps tiny bits of the gold off until the frequency of vibration is just right.

It's a wonder that these magnificent engineered objects are so inexpensive. I'm Bill Hammack the engineer guy.