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The vertical long-period
seismometer designed by James D. Lehman. A copy of the drawing from the
original Scientific American article. |
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Seismometer built by Dr. Joseph J.
Gerencher, Moravian College, PA
Explanatory notes by J. Gerencher
Pictures by James Green. |
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The home-made 18-second-period
horizontal seismometer under its protective Plexiglas case in the Earth
Science classroom/laboratory. The protective case keeps air movements
within the room from affecting the equipment. It also keeps fingers from
touching the equipment directly. |
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The metal table at the back of the Earth
Science classroom/laboratory with all three seismometers on it. |
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Classroom seismometer with case removed
showing frame (left) attached to horizontal boom by guitar string with
turnbuckle for making adjustments. From left to right along the
horizontal boom are the following: inertial mass (lead block), clamp for
attaching guitar string to boom, inertial mass (another lead block),
clamp for holding horseshoe magnet, and Plexiglas blade for oil damping
of swinging motion. Beneath the boom are the amplifier (left) and the
TINI for sending signals to the Czech Republic. The TINI for the
Moravian server is located behind the wall. |
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Adjustment screw on particle-board base.
A double thickness of particle-board, with the two pieces glued
together, is used to try to reduce the base changes with the changes of
room humidity. The table has metal legs and a metal top, again to try to
reduce dimension changes with changes in humidity. |
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View looking down horizontal boom. The
gray shelf holders on opposite sides of the horizontal boom limit its
motion so the entire apparatus does not fall over if it is adjusted
beyond the normal range of motion. These are essential for setting up
and moving the device, but play no part in its normal daily operation. |
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Attachment of guitar string to top of
pipe frame. Vertical bolt is for securing the string; the horizontal
bolt provides a shallow notch to hold the guitar string over the center
of the frame. The period of the seismometer is determined by the length
of the horizontal bolt; the farther forward it is the shorter the period
of the seismometer. This seismometer is adjusted to have a period of 18
seconds. |
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Connection of guitar string to
horizontal rod is made by old chemistry equipment clamp, modified for
this application. Turnbuckle allows the length of the string to be
conveniently adjusted. |
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Knife-edge end on horizontal boom in contact with plumbing fixture.
View looking down from top. The bolt allows the boom to be lifted free
from the plumbing fixture for maintenance. Simple friction is
sufficient to keep the knife edge on the fixture. The fixture has been
filed to a smooth plane on this one face.
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Close-up of knife-edge of horizontal
boom on filed plumbing fixture. Bolt is for protecting edge when
maintaining or moving equipment. |
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Close up of end of horizontal boom
showing pick-up coil and horseshoe magnet and damping blade in beaker of
common motor oil. Ring magnets have been installed onto blade and a
solenoid has been taped (via yellow electrical tape) on the far side of
the beaker to provide the hourly “kick” for the Czech software
development project. A household timer turns on and off the electricity
to a 5-volt power supply which is connected in series to the solenoid
and three light bulbs. When the circuit is on, current runs through the
solenoid and lights the lights. The solenoid repels the ring magnets on
the Plexiglas blade, making a motion of the horizontal boom. An hour
later, when the current stops flowing, the boom drifts back to its
undeflected position, giving another boom motion. This is repeated every
two hours. |
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Close-up view of magnet and coil. The
coils are hand-made and are the most difficult part of the project. They
have about 12 thousand to 14 thousand turns of wire. The coils are wound
onto hand-made Plexiglas core and sides, on a Plexiglas holder and brass
bolts and nuts. All of this is attached to a wood base.
The coil does NOT need to be within the arms
of the horseshoe magnet, but should be sufficiently near the magnet.
There are probably a lot of non-linear distance-related affects in this
portion of the operation. Drifting of the coil relative to the magnet is
always a problem, in particular when you go from the heating season (low
humidity) to the cooling season (high humidity). |
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Close-up view of the oil damping system
and the “kicker” solenoid attached to the beaker by yellow electrical
tape. |
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The amplifier box. |
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The TINI for the Czech software project. |
