As the responses to the geomagnetic field magnitude question were
rather sparse, I should mention that an instrument to measure the ABSOLUTE
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 coil
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.