In a message dated 6/12/00 10:18:52 PM Eastern Daylight Time, 
mlamb1@.......... writes:

<< Actually there is quite a number of PSN email recipients that are also
 interested in astronomy and/or other sciences.  The torsion-pendulum
 magnetometer almost....sounds like it could be along the line of Roger
 Bakers design article in the Amateur Scientist section of Scientific
 American, of (?) January 1999 >>
        
------------------------------------------------------------------------------
-----
Hi Meredith,
    Yes, it is similar but the McWilliams torsion-pendulum magnetometer has 
been around for 20 years and many have been built and are sensitive enough to 
easily produce excellent magnetograms comparable to those made at USGS 
magnetic observatories. It is also a simple device not requiring any laser 
beams or fancy amplifiers like Roger Baker's. This easy to build magnetometer 
is an 8-inch long compass needle suspended on a torsion wire, a 15-inch 
length of guitar string (.008" dia.) from your local music store. A shadow 
vane on one end of the needle shades two Radio Shack Cadmium Sulfate 
photocells from the light of a 12-volt automobile bulb. The torsion wire is 
twisted enough to provide enough torque to make the needle point east and 
west rather than the north and south direction it would rather point. The 
needle is therefore torsion balanced against the Earth's magnetic field. In 
this position the two photocells are placed side by side under the light so 
the shadow vane on the end of the needle shades half of each photocell. The 
photocells are variable resistors and there resistance depends on how much 
light falls on them. When they are equally shaded their resistance is equal. 
They are made half of a Wheatstone bridge and a strip chart recorder or 
computer is connected across the bridge. No current flows so long as the 
bridge is balanced by the photocells being equally illuminated. When a solar 
flare or coronal mass ejection bombards the Earth's magnetosphere with high 
speed particles they generate a current that produces a magnetic field that 
either adds or subtracts to the Earth's ambient field thus changing the total 
field strength. The magnetic needle feels the change and its balance against 
torsion rotates the shadow vane so the photocells are no longer equally 
shaded and their resistance is unequal. This generates a current in the 
bridge and moves the chart recorder which is zero centered. The direction it 
moves depends on the polarity of the bridge current and this depends on 
whether the solar particle flux generates a magnetic field that adds or 
subtracts to the Earth's, thus making a recording that is the familiar 
magnetogram like the ones that you can see at 
<>. If you or anyone else would 
like a copy a Solar Bulletin that describes the McWilliams  magnetometer in 
detail with drawings and a schematic of the Wheatstone bridge, just send a 
self addressed envelope to me at my address below and I'll be glad to send 
you a copy. The photocell displacement transducer might also work quite well 
on a seismograph.
Casper H. Hossfield
PO Box 23
New Milford, NY 10959
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)