Regarding producing a suitable magnet and coil for a seismometer:

Last year I described how I was making a powerful coil/magnet
system for a new broadband instrument, using a rare-earth magnet 
assembly made with bought parts (for about $30) and a hand wound 
formless coil of 1200 turns. (#36 Beldsol wire about $5).

The transducers all have a constant of about 10.5 Newtons/Ampere
(Volts/meter/second). I have made about a dozen for the project
so far. They are quite compact, and suitable for either a force
feedback coil or a simple velocity transducer for a Lehman design.

I can repeat the info if there is a need. I believe that several
PSNers have successfully made several versions. The design certainly
lends itself to simplicity and repeatability.

A few comments though.

A transducer's output has to be described in Newtons/Ampere or
V/m/sec. Any coil/magnet will produce an infinite voltage if you
move the coil fast enough. The force constant is easily determined
by a simple balance setup, where the deflection caused by a test 
weight is balanced by controlling a variable current in the coil.

Bob has suggested letting the wire wind off the end of the spool.
I find that this twists the wire, causing potential management 
problems where the layers have to be formed very uniformly. I made 
a "dispenser" with a shallow box and dowel through the small (1/4 lb)
spool, and a variable tensioner made by passing the wire through a 
post-it note pad, where various weights on top of it control the 
tension. The formless design requires that each coil layer be 
carefully laid up, so I turn the temporary form by hand.  I may later 
clamp down a VSR drill to do the job, and control it with a footswitch 
and electronic brake.

Regards,
Sean-Thomas
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