NOTES 119 



application of X-rays is sufficiently obvious. It has, for example, already been 

 used for the examination of welds, for investigating the effect of aluminium in 

 preventing the formation of blow-holes in cast steel, for the detection of weak 

 points in castings whereby the method of casting may be altered until no such 

 points are visible, and for the examination of iron embedded in concrete. The 

 difficulties to be overcome fall under two heads — firstly, the production of rays of 

 sufficient penetrating power, and secondly the prevention of fogging due to 

 secondary radiations from the material and from the photographic plate itself. In 

 the first place, the penetrating power of the rays {i.e. their wave length) should be 

 adjusted to the thickness of the material to be traversed in order to obtain the 

 maximum effect on the photographic plate, for it is desirable that the rays which 

 penetrate the metal should not succeed in passing through the plate but be 

 absorbed by it. The rays which get through the plate can, however, still be 

 utilised by means of intensifying screens of lead or of calcium tungstate placed in 

 contact with the back of the plate. With lead the electrons liberated by the 

 X-rays intensify the action of the rays on the sensitive film, while the calcium 

 tungstate gives out a blue fluorescence, and this visible light produces its usua 

 effect on the silver bromide. This screen reduces the time of exposure to at 

 least one-sixth, and under particularly favourable circumstances to one-thirtieth 

 of the period otherwise required. To minimise the effects of the secondary 

 radiations which arise when the X-rays are absorbed by ordinary matter it is 

 necessary, when dealing with thick specimens of iron, to surround or embed the 

 specimen in lead (e.g. by piling small shot round it), which must overlap 

 the edges of the plate, to prevent " under-radiation " from its back or from 

 surrounding objects. 



A most important fact in connection with this work is that the relative thickness 

 of the specimen examined and of the fault in it does not matter. Provided only 

 that the rays penetrate the specimen and that the flaw is not thinner than a 

 certain minimum, it will produce its proper effect on the plate, and sufficient 

 contrast for visibility can be obtained by suitable exposure and development. 

 Thus, Dr. Slade estimates that by using the K radiation of nickel with iron it 

 should be possible to detect differences in the thickness of the iron of the order of 

 4 x io -s cm. In practice the tool-marks on the surface are easily visible. At the 

 present stage of its development the process reaches its most striking application 

 in the detection of faults in timber or of faulty workmanship in articles made 

 thereof. At the meeting Dr. (Major) G. W. C Kaye showed numerous examples 

 of faults in aeroplane construction discovered by means of it. The absorption in 

 this case is so small that fluorescent screens can be used for direct visual 

 examination instead of photographic plates — a possibility which makes the process 

 one of ordinary commercial utility. Two of the many photographs shown at the 

 meeting are reproduced on the adjoining plate. Fig. 1 shows an X-ray photo- 

 graph of two brass cartridges taken by Dr. Coolidge and lent to the writer by 

 Prof. Bragg ; the great absorption produced by the fulminate of mercury in the 

 detonator will be noted. Fig. 2 shows one of the photographs taken by Major 

 Kaye and Dr. Knox, and represents front and side views of a laminated wooden 

 aeroplane spar made of three laminae glued together. The external appearance 



to penetrate through 9 inches of that metal. In Germany the Lilienfeld tube is 

 used instead of the Coolidge tube. It requires three separate currents and permits 

 of the independent variation of the heating current and of the driving potential 

 which imparts to the cathode rays their necessary high velocity. No information 

 as to its capabilities appears to be available in this country. 



