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Making a Seismograph |
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A seismograph is a device used for
recording earth tremors. Basically, it is a heavily weighted horizontal rod (pendulum)
suspended from a pole. This rod swings from side to side if the earth shakes. One end of
the rod rests against the pole, |
while the other holds a
pen or stylus. This stylus marks a slowly moving roll of paper. If there is no shaking,
the passing paper is marked with a straight line. If there is a tremor, the paper is
marked with a squiggly line.
The waves reach the seismograph in the following order: First, the P or primary waves,
which are caused by compression of rock, and which travel straight through the earth;
Second, S or secondary waves, which are shear waves caused by rock shaken from side to
side; and third, L waves, which are surface waves caused by rolling motions of the
surface. L waves are the last to arrive and are the most destructive. Since these waves
travel at different speeds, seismologists can pinpoint the origin of an earthquake
(epicenter) by comparing their arrival times.
A simple seismograph, is easy to build and will record local vibrations such as passing
trucks or people walking past it. The text for this model are from Science Equipment, by
William Moore, 1962. |
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| You will need: A steel
pipe, threaded at least at one end, 25 mm by 100 cm, a steel floor flange 88mm in diameter
(should fit the pipe), a steel rod 6mm by 60cm, a steel wire approximately 100 cm long, a
wind-up alarm clock, a piece of wood 20mm by 305mm by 915mm, screws |
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Instructions:
"Begin by screwing the flanged plate to one end of the large board. Next drill holes
through the top end of the pipe for the wire. Also drill a hole for the wire at about 25mm
from the end of the steel rod. The steel rod must be pointed at both ends, so place it in
a vise and file on alternate sides of the rod. After this is done, drill a small dimple
about 150mm above the threaded end of the pipe in which one pointed end of the steel rod
may rest. Attach the wire to the end of the pipe and the end of the rod next, and after
screwing the pipe into position tightly, place the pointed end of the rod in the dimple.
Now wire some heavy weights to the steel rod. These may be lead blocks as shown here, or
bricks, or even sash weights.
The recording device should be prepared next. This is done by removing the glass or
plastic cover on the clock face and cementing a white cardboard disc to the (hour) hand of
the clock. Use a candle or small lamp to cover the disc with soot. Now mount the clock on
a piece of scrap wood, so that the point of the steel rod barely touches the disc at the
nine o'clock position on the dial. Experiment with the position of the clock until the
correct location is found before fastening it permanently. For greater accuracy, be sure
to clamp the baseboard to the bench or table."
Modifications
Once the basic design is understood, it is possible to design and construct a much more
sensitive seismograph. A few tips are:
1.The base should be solid and positioned on the ground. A large paint drum half filled
with concrete works well.
2.The upright support should be very solid. Metal pipe works well. The post should be
anchored firmly in the concrete base. Three or four feet high (90-125 cm) should be
sufficient.
3.Any rigid length of metal can be substituted for the steel rod arm (pendulum) in the
above plan, as long as it rigid enough not to bend when suspended from the upright
support. A thin strip of metal can be folded to make a strong, rigid channel. It should be
about 3/4 long as the upright is tall. The end of the rod that contacts the upright comes
to a sharp point. Ideally, the point of contact on the upright should be a ball bearing,
which can be adjusted in or out for fine-tuning. This point of attachment should be about
150mm up from the base (this will depend on how high your recording apparatus is
positioned).
4. The wire supporting the arm/pendulum attaches to the top of the upright. It is best to
make this attachment adjustable with an arrangement of angle irons, nuts, and washers so
that the wire and pendulum can be easily raised, lowered, and moved from side to side in
small increments. 5. The wire attaches to the
arm/pendulum at about six inches from the upright. The weight should lie near this point
of attachment. A metal can filled with plaster is convenient to work with; the
arm/pendulum can pass through it, and the wire can attach to the can itself.
6. The other end of the arm holds the recording stylus.
What is used here depends on the material being recorded on. A needle works well for
smoked metal, aluminum foil, or glass. If paper is used, pens must be chosen that will
write with light pressure, but will not readily dry out. The type of pens used in plotters
are one option. Whatever is used, it must be remembered that it will be striking the
recording surface very lightly.
7. The clock dial method above works well as a recorder.
The recorder should be firmly set so that it does not get knocked around easily.
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