PURPOSE: To semi-quantitatively measure the magnetic field in the air gap of a magnet.

DESCRIPTION: 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:

where N is the number of turns and i is the current in the coil.

Below are photographs of two gap separations (with the yoke sections touching and separated by a piece of plywood inserted above the coil) and the voltage picked up by the loop for each.

SUGGESTIONS: Put a wooden box under the apparatus to eliminate distortion of the oscilloscope picture by the fringe field of the transformer and the variac.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for further information and derivation of equations.

EQUIPMENT: Demountable transformer, oscilloscope with TV display, variac and search coil.

SETUP TIME: 10 min.