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Doctor science strikes again

By Chuck Newcombe

I've previously discussed how I became involved, at Fluke's request, with the local Imagine Children's Museum several years ago as a science advisor on the use of a Van de Graaff generator to raise the kiddies' hair. In that role, I gained the title "Dr. Science."

More recently, Fluke has followed up with support of a regular monthly museum program called "i-Engineers." In my evolving role, I volunteer to structure projects that allow kids to experience topics that an engineer might encounter, while using hands-on activities to make the topic fun and memorable.

A recent example, rather timely given recent events with earthquakes and tsunamis in the South Pacific, is a discussion of earthquakes and how engineers develop instruments (seismometers and seismographs) to measure them.

I first built a two foot diameter plywood turntable, capable of handling a 300 lb. load, for the kids to stand on. Then, I caused the table to vibrate back and forth by connecting the business end of a reciprocating saw (called a Sawzall by the Milwaukee Electric Tool Company) to a driving arm that rotates the table back and forth over a short distance. As it turns out the experience can be a little disorienting, as an earthquake might be, leading to a discussion of the "Drop, Cover and Hang On" exercise practiced in school drills from time to time.

Now that we have the pseudo "earthquake," what vibrations actually occur in a real one, and how do we measure them?

Two key waves generated by earthquakes are the Primary (P) wave, also known as a Compression Wave, and the Secondary, or Shear (S) Wave. Both waves are detected and measured by a recording seismometer (seismograph), and the difference in their arrival times noted. Since the waves travel at different average speeds, this difference allows one to estimate the distance of the 'quake from the recording station. Drawing distance circles around three recording stations distant from one another, we find an intersection of the circles on a map that corresponds to the location of the earthquake.

For the kids, we used a complex device called a Slinky to demonstrate the travel of these two basic wave types across the floor. The kids actually participate in generating the waves and then observe them as they travel the length of the stretched out Slinky.

Now it's time to tackle the seismometer and how it works.

Newton's First Law of Motion says, "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." In our case, that can be interpreted to read, "An object at rest will tend to remain at rest." Put a mass at the end of a free pendulum, and as the earth moves beneath the device, the device moves while the mass holds its position. All we have to do is sense the movement of the earth beneath the unmoving mass, which we do with a mechanical linkage or an electrical generator.

I built a vertical seismometer to demonstrate the principal. It turns out that many of the kids who experienced our exercise recognized the device as a variation of the primitive electric motor that some of them had built as a home or school science project. In our version, the magnets and coils formed a generator that produces a voltage proportional to the speed and distance that a magnet travels through a coil, sensing the motion of the seismometer around a mass (fishing weight) suspended by a spring.

Finally, we measured the output using one of Fluke's finest - the ScopeMeter 199C. Nothing is too good for our kids, right?

The final step for the kids is to use a weighted paintbrush on a string to paint a simulated seismic wave on a piece of paper on the floor. They take this home, along with their "i-Engineered at the

Imagine Children's Museum badge along with take home information sheets for later discussion with their parents.I didn't need to use the 199C to display this low frequency result -the much less expensive 123 might have worked just as well, and I even used an 87-V DMM to capture the peaks of the waveforms. Oscilloscopes present so much more information, and the Color 199C helps sort out the two waveform displays that are possible. It's all about better communication of recorded events as shown in this simple example with children as the audience.

More information on earthquakes and seismometers:

View recent earthquakes in the U.S. and around the world at Earthquake Hazards Program »

Well-organized, global source of earthquake information: Earth and Space Sciences at the University of Washington »

The University of Nevada at Reno offers info on faults, seismic waves, and details on how to measure the distance to quake: Nevada Seismological Lab »

Purdue University gave me the idea for my vertical seismometer: »

Here's a comprehensive site offered by the British Geological Survey: »