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To measure a magnetic field, a useful item is a 'Hall effect' device.
It usually has 3 pins, for supply, ground, and output, and looks a bit like a transistor. The idea is that the voltage at the output pin changes in proportion to a magnetic field. Under no field, the output is half the supply voltage. If you bring it near a north pole of a magnet, the voltage increases, and a south pole will cause it to decrease. Thus, you can not only measure the strength of the field, but the direction, too.
Are you measuring static fields or alternating fields? This will affect the type of circuitry you need for measuring the output voltage. In other words, are you measuring a DC or an AC voltage?