Seismograph

Challenge
To make a seismograph, capable of measuring glacial movement.

What is a seismograph?
A seismograph is an instrument that records earthquakes. The output that it produces, showing the earth’s movements, is known as a seismogram.

How do they work?
To make a seismograph, you need to get one thing to move with the earth as it shakes, and something else to stay as still as possible relative to the earth. Not easy when the whole earth is moving! Ideally, you’d have a pen, suspended in space, not influenced by the earth at all, marking onto paper that is fixed to the earth.

But that’s impossible. The fixing of the pen will have to involve the earth in some way. But if the pen is suspended from a pole that’s attached to the earth, the pen can swing with some freedom. By attaching a big weight to the pen, that’s also suspended, the added inertia (inertia is a measure of how reluctant something is to move. Heavy things, with a big mass, are harder to move than lighter things. Also, once they’ve started moving, they are harder to stop. So things with a big mass have a lot of inertia, while things with a small mass have low inertia) means the pen moves more slowly than everything else, lagging behind the movement of the earth. This way you can make a seismograph.

History of seismographs
In China, in AD132, a man named Zhang Heng made what was perhaps the first seismograph. It was a copper vessel carrying eight dragon heads, which were positioned above eight frogs. Each dragon carried a ball in its mouth. An earthquake shaking this device strongly enough would dislodge the ball from a dragon’s mouth which would fall into the open mouth of the frog directly below it. By finding out which frogs contained balls after a strong earthquake, Zhang Heng could work out how the earth had moved in response to the earthquake.

How to make a seismograph
What you’ll need:
A base to put it all onto (wood is easy to use)
A pole to hang the pen and weight from
A rod that hangs from the pole and is free to swing sideways
A pen to attach to the rod
A heavy weight to attach to the rod (eg a brick)
Strong wire, rope or twine (that doesn’t stretch too much)
Some way of moving paper at a steady speed (eg round bits of paper stuck to the hand of an old clock)
Nails, hammer, drill, glue, clips.
Small flat piece of metal to use as pivot point for rod.

Build the seismograph according to the diagram, but consider these points if you make it yourself:

The pole needs to be attached very firmly to the base (and the base attached very firmly to the ground).
Many strings and wires stretch, so experiment. Be careful when the weight is hanging from the wire, taut wire that breaks suddenly can hurt if it hits you.

Sharpen the end of the rod, or have the sharp end of a screw sticking out of the end. Make a dent in the piece of metal to use for the pivot point (by putting the pointed end of the rod in). You don’t want there to be too much friction here, but you want the rod to stay up.

If the small piece of metal is attached to the upright pole with perhaps a nail, you can move it from side to side (so making the pivot point move left or right) which is useful for centering the pendulum rod.

Attach the string to the rod to easily change the place that the string is attached to the rod. The distance from the pivot point to the string is something you can adjust to find where you get a stable arrangement. The distance along the pole between the string and the pivot point (call it ‘A’) should be about ¾ of the distance between the pivot point and where the string is fixed to the swinging rod (‘B’), so if B is 1m long, use an A of about 75cm.. You can either have the pen pointing horizontally, and the paper mounted vertically, or attach the pen so it’s vertical and have it pushing very gently down on the horizontal paper.

To make the paper move round at a steady speed, you can attach a round bit of paper to the hand of a clock. Using the minute hand means you can get readings over an hour before you need to replace the paper. The hour hand will give readings over 12 hours. The second hand will give readings over a minute. We took readings over an hour, and mounted the paper onto a wooden disc with a slit cut in it, that just fitted over the minute hand, which carried the disc around.

Limitations of this seismograph
You’ll need quite a hefty movement of the earth to be able to record anything on this seismograph. It relies on the earth moving far enough, relative to the suspended pen, to actually make a detectable reading.

In the UK we generally don’t get much seismic activity, so this is the kind of thing to try when you’re next in California or even New Zealand.

Interpreting your seismograph
Any small movement in the line that the pen traces out indicates that there has been some movement in the ground. We managed to measure some movement when they put the seismograph on a bridge. We also got tiny readings when we jumped next to the device. The helicopter bringing the other scientists back from the mountain also pushed the pen right off the paper. If there had been an earthquake, we might have detected it!

See if you can get the Earth to move underneath your seismograph. Jump next to it, or drop heavy objects nearby. Try it out on different surfaces.

Modern day seismometers
Seismometers do what seismographs do: they measure earth movements.
Scientists studying seismic activity are using rather more sensitive devices that the one we made. Modern seismographs are able to detect tiny movements in the earth’s surface, usually using electromagnetism.

If you move a magnet inside a coil of wire, it creates an electric current. It’s easy to amplify an electric current, so even tiny movements can be detected. Today's seismometers measure the amount of electrical energy needed to keep a mass centered in its housing when the ground is being shaken.

Not only can modern seismometers detect tiny movements in the earth, they can record a wide range of seismic signals, from very low to very high frequencies