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Subject: a possible seismometer?
From: Randall Peters PETERS_RD@..........
Date: Mon, 08 Dec 2008 16:50:11 -0500

I've been contemplating an instrument based on a piece of equipment we
use in the physics department to teach hydrostatics.  It is a long tube
that is sealed on one end, and the open end is immersed in a larger tube
filled with water.  The smaller tube achieves an equilibrium position
determined by its weight, overall length, and diameter of the internal
air column.  If one lifts the inner tube and releases it, it oscillates
in simple harmonic motion with a period greater than one second.  Also,
because of the water, the motion is dampened.  The motion depends on the
compressibility of the trapped air.
    I've done an idealized calculation in which the water would be
replaced with mineral oil and the plastic tube of the apparatus with a
thinwall metal tube of length in the neighborhood of one meter.  It
appears that the damping would be close to optimal and the period just
under two seconds for the diameter of the inner tube being one
centimeter and that of the outer tube holding the mineral oil about two
    The inner tube should be constrained (perhaps with long triaxial
threads, so that the motion is strictly vertical.
I will be interested to know if any of you have considered such an

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