efficient and extremely fast way of sorting alloys and or identifying individual alloys the output signal from the summing amplifier 30 can also go into a radiation thickness gauge to correct the thickness reading for changes in composition a typical radiation thickness gauge comprises a source of x rays on one side of the material and an x ray detector on the other side the radiation which is not absorbed is transmitted through the material and the transmitted beam is compared with a predetermined value based upon measurements of a standard material of known thickness if the thickness increases the transmitted beam decreases thereby giving a lower reading this reading is then compared with the standard usually in a mini computer which forms part of the function circuit and a correction based on a curve of the standard for different thicknesses is fed forward to the rolls to bring the thickness in line with what it should be this x ray thickness gauge is diagramatically illustrated as part of function circuit 34 fig4 the output signal from the function circuit 34 can then be used to control the screwdown on rolls 36 so as to maintain a constant thickness of the product as stated a change in composition also effects the density and thus the transmitted beam therefore the summed signal from amplifier 30 is first used to identify the material as in the earlier embodiment this identification of the material establishes the specific curve which is to be used for comparison and correction purposes to determine the signal which is fed forward to control the rolling process another way in which the summed signal can.