Erich Kern writes

> Several years ago, I  attempted
> making a magnetically shielded enclosure to test our magnetic sensors using 1"
> I.D. steel pipe 18" long with pipe caps which turned out to be completely
> ineffective. When I mentioned this to the late Richard Noble of Speake & Co.,
> he told me the only effective shielding to do zero field tests was to fabricate
> concentric Mu metal shields which had been annealed AFTER fabrication and
> separated by an inch or more. This is very expensive. The alternative is to
> simply orient the sensors east-west outdoors, away from the building and road
> traffic.

You got good advice, but there is a cheaper way to do it. Plain soft iron
is also highly permeable, but hard to work. When I worked at Geometrics
we spend a pile of money on a mu-metal shield can for working on cesium
magnetometers, and it was three layers of about 0.060" mu-metal, separated
from one another with stiff felt. Checking for noise outdoors oriented
east-west might work for vector (single-axis) instruments of moderate
sensitivity, but not for (a) total field magnetometers or (b) instruments
of high sensitivity. The problem is that even with perfect alignment to
the east-west null, any motion of the magnetometer in the earth's field 
translates into a change in flux, which becomes an output signal. In other
words, you've made a seismograph. 

> The Mu metal does not "stop" the magnetic flux, it conducts it around the
> object within. It's curious that most people seek an analogy with nuclear and
> microwave radiation (very much higher in frequency) rather than an
> electrostatic field which it most resembles. I used to do shielding
> effectiveness tests on very large enclosures, and more voids in the shielding
> were evident with a loop antenna at 200 KHz than with an E field dipole at
> microwave frequencies.

You got it. For the other readers, note that a loop antenna works by
magnetic coupling, while a (resonant) E field dipole intercepts both 
the magnetic and the electrostatic field. An E field probe intercepts 
almost exclusively the electrostatic field. 


-- 
David Josephson / Josephson Engineering / San Jose CA / david@.............
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