Hi Jon,

  I looked at your photos, and I think more work is needed.

  Be sure that none of the materials used in the boom and its parts
are magnetic.

  DO SOMETHING ABOUT THE WIRE LEAD TO THE COIL!!! You cannot just
leave it hanging loose from the boom. Cut it near the lower pivot,
secure it to the boom, and join it with coils of fine copper wire to
terminals on the frame. The terminals should be located as near to the
lower pivot as possible.

  TURN THE DAMPING VANE NINETY DEGREES. Then you can adjust the
damping by adjusting how far the vane protrudes into the magnet. As it
is arranged now, you can only adjust the damping by changing the field
gap of the magnet.

  Think about cross-bracing the uprights supporting the upper pivot in
order to reduce side sway, which could cause instability. An "A"
shaped upright frame would be the best for lateral stiffness.

  Be sure to mount the device on a base in solid contact with the
ground. Avoid any compliant material between the feet and ground. Long
period horizontal sensors are very tilt sensitive, so expect trace
excursions if you walk anywhere near it. Locate it inside and as far
away from human activity if you can.

  I do not like to see the coil and damping vane dangling down so far
below the pendulum.

  I cannot comment on your pivots, because there is not enough detail
shown. My personal belief is that the materials should be very hard,
polished, and preferably of dissimilar composition -- for example,
hardened steel mated with sapphire. Hard material resists flat spots,
and the use of different materials resists stiction from molecular
attraction at the interface.

  If you use balls and flat plates for pivots, the balls should be
attached to the frame, not the pendulum. This is to avoid shifting of
the hinge line if the point of contact of the ball with the plate
should slip.

  Similarly, if you use crossed rods for pivots, the axes of the rods
mounted on the frame should lie in a vertical plane.

Bob
PSN Station REM
http://bobmclure90.googlepages/home

~~~~~~~Original Message~~~~~~~~
Subject: New Lehman seismometer
From: jonfr@......
Date: Thu, 30 Apr 2009 19:59:23 +0000

Hi all

I got a new lehman seismometer built from the old one, that was not
working. The new one is bit more complex and more adjustable. Now I just
have to figure out how to get the seconds down to 10 to 20 seconds.

Here are pictures.

http://www.jonfr.com/myndir/v/geology/lehman/p4300001.jpg.html
http://www.jonfr.com/myndir/v/geology/lehman/p4300002.jpg.html

Regards.

Jon Frimann
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