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Hi, Dr. D. Hi, kids.

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Is this the seismograph you're building?

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That's right. I'm almost finished.

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This seismograph is not very sensitive, but it does work.

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It measures horizontal motion.

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So you actually use a peanut can and an old brick?

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Yeah, sure. Let me start from the beginning.

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The seismograph measures the shaking of the Earth.

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To measure the shaking, we take advantage of Newton's first law.

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I think I remember. Isn't it also called inertia?

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I'm impressed. So what is inertia?

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It means that when an object is at rest, it stays at rest.

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And when an object is in motion, it remains in motion.

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That's right. This brick has a lot of inertia, so it's hard to get moving.

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When the Earth shakes, the base of the seismograph will also shake.

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But the brick, which is not tied to the base, will not.

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The can over there records the vibration.

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Can we try it?

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Sure. Let's make an earthquake by shaking the table.

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It really works. Look at the waves. Did we do that?

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You sure did.

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Here's a different kind of seismograph. It measures vertical motion.

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It looks really different.

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Because it has inertia, this magnet hanging on the spring tends to stand still when the table moves up and down.

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A magnet moving through a coil of wire produces electricity, which this meter records.

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Can we try this one too?

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Sure.

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All right. But here's a sense of the seismometer that hooked up to this computer.

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That graph looks a lot like the seismogram that Mr. Lau showed us.

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The harder I hit the table, the larger the signal. This is really cool.

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And look at how it vibrates up and down.

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Because it produces an electric signal, it is easy to amplify.

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Even a small vibration can be shown on the computer.

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Now, I can't loan you this device.

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But why don't you take my wooden seismograph back to the treehouse and see if it can record another tremor?

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I have a friend at NASA's Jet Propulsion Laboratory in Pasadena, California.

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He uses satellites to measure the motion of the Earth's crust.

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You might want to talk to her in your quest to become earthquake experts.