A galvanometer in this context is a small mirror attached to a coil of =
wire
in a magnetic field. When a current flows through the coil, it twists =
the
mirror (think of a moving coil voltmeter with a mirror instead of a
pointer).

These units were used in seismic observatories. A focused narrow light =
beam
would shine on the mirror and be reflected onto a rotating and =
translating
drum covered with photographic paper. By making the light path long (say =
two
or more meters), you could achieve significant optical magnification.

The long period seismometers generated the small current fed to the
galvanometer. Combining all the pieces produced a sensitive seismograph
before the days of 24-bit converters or even electronics. Once a day,
somebody would change the paper on the drums and develop the previous =
day's
record.=20

United Electrodynamics in Pasadena was an early manufacturer of seismic
instruments. They were bought by Teledyne in the 50's and merged into
Teledyne Geotech (now just Geotech). Some of the folks at UED left and
started Kinemetrics in the early 60's.

Thus ends your history lesson for the day. I have no idea how one could =
use
these fine instruments today.

Doug Crice          http://www.geostuff.com
Wireless Seismic            http://www.wirelessSeismic.com
12996 Somerset Drive                phone 1-530-274-4445
Grass Valley,  CA  95945  USA    fax 1-530-274-4446
=20

-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@............... =
On
Behalf Of BOB BARNS
Sent: Thursday, August 11, 2005 7:11 AM
To: psn mail
Subject: ebay auction

Hi gang,

    GALVANOMETER Seismograph Earthquake Physics
LEHNER GRIFFITH GL-261- Used to Measure Earthquakes	Item number:=20
7537798905	ends Aug 17

  I don't know if this is of any interest but a period of 99 secs.=20
caught my eye.
Bob

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