A disc of extremely permeable material called a pie gauge has non-ferromagnetic material dividing it into four, six, or eight parts. The divisions act as manufactured flaws that project outward in various ways from the core. The gauge has a diameter of 3/4 to 1 inch. The gaps between the pie parts made of low carbon steel must be no wider than 1/32 inch. The parts are copper-plated and brazed in a furnace. The test piece is magnetised and the gauge is set on top of it with the copper side facing up. The signals give the inspector information about the magnetic field’s orientation once particles are applied and any surplus is eliminated.
