The article describes an instrument for determining the percent content of ferromagnetic phase in a nonferromagnetic matrix, e.g., of the ferritic phase in a nonferromagnetic matrix, e.g., of the ferritic phase in welding seams of austenitic steels. The instrument has two sensors, a pencil-type sensor for measurements directly on the seams and a fixed sensor for measuring the ferritic phase in specimens cut out from the zone of the seam.