In a message dated 04/02/00 01:12:40 GMT Standard Time, pho@........... 
writes:
<< I was looking around on the web and found a good source for magnetic 
sensors,
 and others at: http://www.vernier.com  Peter >>
    
    This is a long list of quite interesting general purpose school level 
sensors. The audio sensors do not have audio range or sensitivity needed for 
Infrasound measurements which range from ~4 Hz to 0.02 Hz and at 
sensitivities down to 0.01 micro Bar. A specialised range of sensors is 
required, which can be made, but they are expensive to buy.
 
    The magnetic sensor listed is of the Hall effect type and does not have 
the very high stability needed to detect and measure the changes in the 
earth's magnetic field at parts per million and below.
    CapAAVSO@....... describes a torsion bar magnetometer by McWilliams, 
which will work at this level. There was also a "Jam Jar" magnetometer 
described many years back in popular science articles. It was built entirely 
inside a fruit bottling jar for protection against air motion.
    Flux gate or proton resonance sensors are usually used for professional 
work. Fluxgates of some ferrites can be used, but very soft magnetic alloys 
like mu metal may be better. Two rods of the magnetic material are each wound 
with a magnetising coil. They are connected so that the windings oppose and a 
sense coil is wound over the pair. When the coil is energised at a frequency 
F, the sense coil output is at 2xF. An external magnetic field will cause one 
bar to reach magnetic saturation before the other, producing a small 
difference signal. This signal is filtered and is then passed through a phase 
sensitive detector, enabling changes at ppm level to be measured. Ring and 
tube sensors are also available. See site for details of suitable fluxgate 
sensors:- http://fqs.dconn.com/Electronics/sensors.htm 
    Descriptions of amateur constructed fluxgate equipment have been 
published. Magnetic field changes may be of interest to Ham Radio operators 
and to astronomers.

     Regards Chris.

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