The Berthold indicator is designed to determine the strength and direction of the selected magnetizing setup and the quality of fluorescent or colored magnetic particle suspensions.
When placed on a magnetized test piece, magnetic lines pass through the sectioned iron cylinder. The cuts on the iron cylinder will be visible when magnetic powder or fluorescent magnetic solution is applied. Maximum indication direction is achieved by rotating the sample about it's axis. The field direction is perpendicular to the flaw at maximum indication. Magnetizing efficiency, penetration and quality of the fluorescent oil suspension can be determined by slowly turning the outside ring of the field indicator, increasing the distance between the thin brass plate and the test piece. The amount of lift-off at the point where the indication just appears gives a measure of magnetic field test efficiency.