Measure the thickness of the non-magnetic covering layer (such as aluminum, chromium, copper, enamel, rubber, paint, etc.) on magnetic substrates (such as steel, iron, alloy, and hard magnetic steel, etc.) using the sturdy method, and use the eddy current method to measure non-magnetic substrates The thickness of the non-conductive coating on metals (such as copper, aluminum, zinc, tin, etc.) (such as enamel, rubber, paint, plastic, etc.)