Wednesday, February 23, 2011

Making stuff dumber

A friend recommended the Nova series "Making Stuff". According to the series description it is a four part series that explores the materials that "will shape our future." So I stuck it on the old TiVo list and this past week watched the beginning of "Making Stuff Smaller" with my son. It was a torturous reminder of why I stopped watching Nova, and after about 15 minutes we gave up.

Now, I'm not the most brilliant guy in the world, but I am a science geek. Is it too much to ask that the science in a science show should be correct? In the 15 minutes I saw, there were numerous examples of wrong "stuff." Let's look at one example, the invention of the pocket watch.

Prior to the 15th century clocks used pendulums to count the passage of time. They were replaced, according to the show, by the main spring, which turned a gear. But wait! The mainspring didn't replace the pendulum, it replaced the weights that powered the pendulum clock. The thing that replaced the pendulum was the flywheel.

You see, a clock needs an oscillator to provide a steady time base. Every oscillator has two forces, which vary over time. One force drives the system out of equilibrium by putting energy into it, and the other force returns the system to equilibrium. In a pendulum clock, the energy provided by a falling weight is used to swing the pendulum to one side, and gravity pulls it back toward the center. The inertia of the moving pendulum and force of gravity make a system that has a very definite frequency, which is useful for keeping time.

In a mechanical watch the mainspring provides energy to a flywheel called a balance wheel, which has a spring that returns it to the starting position. Like the pendulum, the inertia of the balance wheel and force of the spring control the frequency of the oscillator.

The mainspring was important for creating watches. A falling weight isn't convenient to carry around, and any movement will change the direction/force of the weight. What the mainspring did which was unique was provide a (nearly) constant force. Most springs have a force which is proportional to the amount the spring is compressed.

Likewise, the balance wheel replacing the pendulum was an important feature for much the same reason. However, a scientist (or even a science show host) should know the difference. It's like saying the gas tank replaced the horse, in the invention of the car.

But it gets worse. They the go one to say that the mainspring was replaced by the quartz crystal. Whoa! In a quartz watch it is the battery that replaces the mainspring. The crystal replaces the balance wheel. So we're back to taking about the oscillator, not the power source.

First off, they skipped over an entire generation of watch technology, the tuning fork. In the 1960s, Bulova came out with the tuning fork watch. A transistor oscillator was regulated by a small tuning fork that vibrated at a determined frequency. In this case, a magnetic field pushes the tuning fork and the springiness of the metal in the tuning fork returns it to equilibrium.

The tuning fork was replaced later by the quartz crystal. Back to the show. According to them, quart vibrates when an electric current is passed through it. Wrong again! Quartz is a piezoelectric material, which means it changes shape (slightly) when a voltage is applied across it. When that voltage is removed, the quartz returns to its original shape. Like the pendulum, and the balance wheel, the time it takes to return to equilibrium determines the frequency of the oscillator.

According to the show, the quartz crystal chops time into "millionths of a second". Wrong again! The quart crystal in digital watches typically oscillates at a frequency of 32,768 Hz, or oscillations per second. There are 2 reasons for this. First off, each oscillation requires energy. A high frequency crystal would drain your watch battery in weeks. Secondly, it is very easy in the digital world to divide by 2, and 32,768 is 215 so simply dividing by two 15 times gives a time base of 1 second.

What about those watches that do 1/100th of a second? They use the 1/256th and signal and fudge it by combining the results of when dividing it by two and three. Who's going to notice that their stopwatch is sometimes slightly off in the 1/100s digit?

To make matters worse, the show misses the whole point of the quartz crystal. They claim the reason to use quartz is because it is cheap. It is that, but the real reason is accuracy. Every material has a property called CTE, or coefficient of thermal expansion. This is a measurement of how much the material expands or contracts when the temperature changes.

Prior to the quartz crystal oscillator, every watch or clock suffered from this problem. The arm of the pendulum would get longer with increased temperature, and it would swing slower, making the clock lose time. When it was cold, the pendulum would contract and the clock would run faster. The amount of change might be very small, but since it is affecting the rate of the clock, over time the errors would accumulate. Likewise, the balance wheel would increase or decrease in diameter, which made it run slower or faster. The tuning fork's length would change, making it ring at a different frequency.

The cool thing about the way the quartz crystal oscillates is that the frequency is related to the ratio of its length to width. Assuming the entire crystal is the same temperature that ratio will be the same, no matter what the temperature.

The show didn't appear to be getting any better, as the next segment started with a completely fallacious description of semiconductors. I just had to turn it off.

The sad thing is that these are really cool things, but the makers of the show spent more time making fancy graphics, when an hour with a textbook (or 10 minutes of googling) would have given them a show that actually taught something.

[N.B. Since I started writing this I had time to watch "Making Things Cleaner" which is about energy conservation - it was just as good as "Making Things Smaller". Ugh.]


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