The Severn Gage comes in two models:

LOW MU PERMEABILITY INDICATOR Used to measure the magnetic permeability of weakly magnetic materials such as Stainless Steel. Permeability is a measure of a material’s ability to be magnetized.

FERRITE INDICATOR Used to measure the Ferrite content of a material such as a Stainless Steel weld. It is used extensively to measure the soundness of welds to Stainless Steel pipes in nuclear power plants and food handling equipment. The Ferrite Indicator comes in two types of calibrations: Percent Ferrite and Ferrite Number.