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Re: Measuring The Geomagnetic field

On Tue, 7 Oct 1997 18:09:52 -0700 Leigh Palmer <> writes:
As the responses to the geomagnetic field magnitude question were
rather sparse, I should mention that an instrument to measure the
value of this field is well within reach of a high-school student
- perhaps as a science fair topic.

The method uses the proton spin precession magnetometer.
A small plastic bottle filled with distilled water, with an external
arranged to resonate near 2 kilohertz, feeds a low noise audio amp
and a
loudspeaker or a frequency counter.

The bottle is arranged east-west.
The water is 'polarized' with a DC current through the coil for a few
seconds, then the coil is disconnected from the current source, and
connected to the high-gain audio input.

The formula which relates the audio frequency ( which fades out in a
second or two,) to the absolute magnitude of the field is left as an
exercise for the professor.

It happens I was around when this experiment was first performed by
Erwin Hahn at Cal. It is not nearly so easy to do as Brian indicates.
Hahn had to hang his bottle high above the ground between LeConte
Hall and the Gilman Hall across the way to achieve a sufficiently
uniform magnetic field over the volume of the bottle to see the
effect. It seems to me (though I am unsure of this) that he didn't
use distilled water.

The most famous thing about the room in Leconte from which this was
done is that it was in that room that the first murder in campus
history took place. A technician* working for Hahn during the
evening was shot by the irate husband of a woman he had become
involved with while rooming in their house. The husband, a chemist,
got off with five years, usually meaning twenty months.


*Sex and violence in phys-l! It had to happen eventually. The
technician had been my lab partner in an optics course when we were
both undergraduates.

I didn't even know it was illegal. I can call someone in a couple of
hours and get the answer - I hope. I just paged through Sommerfeld. Why
would I think it would be in there?