The operation of the Indicator is based on the mutual attraction of a permanent bar magnet to a known standard and an unknown material. In use, an Insert is screwed into the top of the case. The magnet is then attracted to the Insert by a force dependent upon the Insert's designated permeability. The end of the magnet projecting from the opening in the bottom of the case is then brought in contact with the material being tested. The Indicator is moved away in a direction perpendicular to the contact surface. If the material being tested has a permeability higher than that of the insert, the magnet will first break contact with the Insert as the Indicator is moved away. On the other hand, if the permeability of the material being tested is lower than that of the Insert, the magnet will first break contact with the material as the Indicator is moved away. Thus, by interchanging inserts, it is possible to bracket the permeability of the material being tested.