The high penetration by X-rays through material and the variation of absorption by the material with change in thickness naturally leads to the well-known technique of radiography, both medical and industrial. Castings and welds are routinely `X-rayed' to check for blow-holes or cracks. Because the results are recorded photographically, good reviews of the technique are available from the manufacturers of X-ray film, e.g. Kodak, Ilford, Gevaert.
This technique can also be used to continuously monitor the thickness of rolled sheet and foils and also of plating on sheet metal. All that is required is a source of X-rays on one side of the moving sheet and a Geiger tube on the other. Any variation in thickness will immediately register as a variation in count-rate. The signal could be used via a relay to automatically correct the fluctuation without intervention by any person.
Radiography is in everyday use in many industries. It is used for examining steel-banded tyres, for testing welds, for checking heating elements and for many other purposes: the possibilities are almost limitless.
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