Meridith, I think this particular magnetometer has one or more small 
magnets attached to a hair-like quartz fiber held
vertically and under tension.
Ambient light (coming in from that long frosted slit on the top and side of 
the instrument) shines on a tiny mirror which
is also attached to the quartz fiber.  Any perturbation of the magnetic 
field causes the magnet(s) and the mirror to move
and thus moving the reflected light beam on a scale.  The scale is read 
through the lens at the top of the instrument.

If the quartz fiber is broken, the instrument is useless.  I haven't found 
a source for them.

Bob Hammond
apsn





At 09:25 AM 5/4/2002, you wrote:
>Bob & Bob and/or all,
>
>It looks to have a optical lens atop the unit, that perhaps
>has "grid marks" somewhere for a value reading?  Aren't
>magnetometers/gravimeters essentially (for the age of the
>instrument), "simply" (likely a understatement?), a vertical
>mass on a special spring?
>
>Meredith Lamb
>
>Hammonds wrote:
>
> > Bob, it is indeed a Sprengnether magnetometer.
> >
> > Bob Hammond
> > http://apsn.awcable.com
> >
> > At 06:08 AM 5/4/2002, you wrote:
> > >Hi gang,
> > >   Ebay item # 865745339 (ends 5/10) is advertised as a Sprengnether
> > >seismometer.
> > >I don't think that is a seismometer but may be a magnetometer or a
> > >gravimeter.
> > >   What is it?
> > >Bob Barns
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