J6-21
Semi-quantitatively measure the magnetic field in the air gap of a magnet.
Using a magnet with a square yoke, separate the top yoke member, creating two gaps in the yoke. Into one gap place a small search coil; into the other place a non-magnetic shim. As the gap length h is increased the field B(gap) decreases:

B(gap) = mu(0) N i / h, where N is the number of turns and i is the current in the coil.

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