Of course a force equilibrium is necessary for a seismometer to operate, re=
quiring for a vertical instrument that the weight of the inertial mass plus=
 associated parts be balanced by an equal and opposite force provided by an=
 element such as a (i) spring, or (ii) fluid through buoyancy, or (iii) mag=
netic force involving feedback to eliminate an otherwise unstable equilibri=
um because of Earnshaw's theorem.  And if really sophisticated, that upward=
 force can be due to the Meisner effect (repulsion) involving a superconduc=
tor.  There is even the possibility of a superconducting force that is one =
of attraction rather than repulsion, involving the 'suspension' effect.  Bu=
t no matter what the source of the balancing force, sensitivity is governed=
 by the potential energy function that fully describes the physical propert=
ies of the opposing force pair.  The magnitude of the restoring force that =
brings the mass back to equilibrium after a disturbance is obtained by taki=
ng the derivative of the potential.  The more shallow the potential, the lo=
nger is the period, and the sensitivity is proportional to the square of th=
e period for such a system.  One possible means for obtaining a shallow pot=
ential and thus high sensitivity is based on the physics of an archer's com=
pound bow.  The following link gives information (including photos) of one =
that was exhibited at the 'broadband conference' several years ago. =20
http://www.iris.edu/stations/seisWorkshop04/PDF/CompoundVerticalSeismometer=
..pdf
There were some discussions of this approach here on the list-serve about t=
hree years ago; but perhaps the concept is worthy of a re-visit--especially=
 for new readers of the ideas we generate.
For one wanting additional physics to what can be gleaned from the site ind=
icated above, there is also an article at
http://physics.mercer.edu/petepag/combow.html

Randall Peters

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