May 8, 19 19] 



NATURE 



i95 



forcings and ingots has received attention, and, pro- 

 vided that the thickness is not too great, the method 

 works well. X-ray scrutiny has also suggested im- 

 provements in methods of casting and welding, as 

 well as modifications in the composition of alloys, 

 with a view to the surer production of sound 

 castings. 



Heavy alloy steels, such as tungsten steels, are, by 

 reason of their greater opacity to X-rays, readily 

 distinguishable from carbon steels. The method has 

 also been applied, with, however, little or no success, 

 to the detection of hair-cracks in steel castings. 

 These cracks (which are of the order of i/iooo in. 

 across) have caused great trouble during the war in 

 connection with the crank-shafts of aeroplane engines. 

 The onlv way of attacking the problem would be to 

 send the rays along the direction of the crack, but 

 the diflficulty is that these cracks refuse to confine 

 themselves to one plane ! 



So far as the thickness of steel is concerned, several 

 workers have taken radiographs through about 2 in. 

 of steel, and this figure may be taken as the practical 

 limit at the moment. Not that greater thicknesses 

 have not been penetrated, but the exposure becomes 

 intolerablv long. In the case of aluminium and its 

 allovs, thicknesses of 4 in. or .1; in. have been radio- 

 graphed with ease. Incidentally, the method is very 

 sensitive to minute differences in thickness — for 

 example, the tool-marks used to face specimens are 

 often clearly shown in radiographs of metals. 



The voltages normally employed have ranged 

 between 100,000 and 150,000 volts, and the currents 

 through the tube from 4 to 15 milliamperes. The 

 question of protecting the operator in this work needs 

 particular attention. 



In steel examination there is no possibility of using 

 the fluorescent screen. Practically all workers have 

 used photographic methods, and, furthermore, have 

 been driven to adopt methods of reinforcing the image 

 on the plate by the use of intensifying screens, metal- 

 backing, or other devices. Pilon and Pearce have 

 obtained good results with photographic films sensitised 

 on both sides and sandwiched between two intensifying 

 screens. They found it possible to determine a thick- 

 ness of i/io mm. through 45 mm. of steel. All 

 workers have found it important to cut out all 

 extraneous radiation. 



The X-ray method of examination is naturally very 

 useful in examining explosive objects — for example, 

 the details of the internal construction of torpedoes, 

 shells, fuses, bombs, grenades, and cartridges. Air- 

 craft construction demands both workmanship and 

 material of the highest class, and a new grade of 

 timber is now specified for this work of a quality 

 such as has never been demanded previously. Knox 

 and Kaye have turned the X-rays to account in 

 Inspecting aeroplane timber parts and plywood^ for 

 faults which cannot be seen by ordinary visual 

 examination. Concealed knots or gum-pockets, bad 

 gluing, or poor workmanship are readily revealed. 

 Only soft rays are necessary, and the great trans- 

 parency of wood permits fluorescent-screen examina- 

 tion — a necessity for routine inspection — and allows 

 any thickness likely to occur in practice to be radio- 

 graphed readily. 



The motor rnanufacturer has radiographed carburet- 

 tors and magnetos while in operation, and so has 

 been enabled to detect elusive faults. The Hadfield 

 Research Laboratory, which has done much work 

 on radio-steel examination, has extended the method 

 to the scrutiny of carbon electrodes for electric steel 

 furnaces. The Post Office has used the rays for 

 testing the amount of mineral matter in gutta-percha. 

 Woolwich Arsenal has also used the method. Radio- 

 graphy would doubtless prove to be a convenient 

 NO. 2584, VOL. 103] 



means of detecting hidden corrosion in metals — for 

 e.xample, in gas cylinders, in ferro-concrete, or in the 

 armouring of cables. Mention should be made of the 

 coming importance of stereoscopic radiography. 



There is one other and entirely different way in 

 which X-rays may supplement the radiographic 

 method of examining material. Prof. VV. H. Bragg, 

 to whom the subject owes so much, has shown that 

 the X-rays enable us to examine in detail the nature 

 and extent of the crystallisation of a body. Now it 

 appears to be the case that there is little in Nature 

 which is not crystalline to a greater or less degree, 

 and, further, it is certain that crystalline structure is 

 of first importance in determining the qualitj' of 

 certain substances such as steel. A large field of 

 research is here indicated. 



We do not anticipate any startling developments in 

 the use of X-rays for the examination of steel until 

 the present apparatus for generating X-rays has been 

 vastly improved. We are led to inquire what part this 

 countrv has played in the past in the development of 

 either the high-potential generator or the X-ray tube. 

 The answer is not very gratifying. The British 

 generator is almost always an induction coil of which 

 the present-day model differs but little in essentials 

 from its predecessor of Spottiswoode's day, except 

 that it is capable of a "fatter" spark and greater 

 output generally. It breaks down less frequently 

 owing to closer attention to the insulation of both 

 primary and secondary coils. But what of design? 

 How many British coil-makers employ a designer 

 who can honestly say that he is not working mostly 

 empirically, by trial and error, by "hit and miss," or 

 whatever you like to call it? The fact is, the man 

 who could do things any other way — by reasoned cal- 

 culation and experiment — has so far not had it made 

 worth his while to work at the subject. Moreover, 

 how and where are men to be trained in the ground- 

 work of the subject? In how many university physical 

 or electrotechnical laboratories does the matter receive 

 even the smallest attention? 



What would prove to be the result of reasoned 

 investigatory work on the induction coil? Compared 

 with other types of high-tension transformer the 

 present-day induction coil is not efficient, and the 

 chances are that it never will be. At any rate, our 

 American cousins have come to that conclusion, and 

 are concentrating on closed-circuit, Interrupterless 

 transformers which can operate with any commercial 

 .A.C. supply, and are generally used in conjunction 

 with some type of hot-cathode "rectifier" to suppress 

 the "inverse" phase of potential. The British answer 

 has mostly been to point out that the sinusoidal 

 potential wave is not so efficient an X-ray producer 

 as the peaked wave of an induction coil. Of the 

 degree of practical importance of this difference we 

 have no experimental knowledge. It is probably on 

 a par with the oft-repeated, but untrue, statement 

 that a Coolidge tube is not so good as an ordinary 

 gas tube for securing first-class radiographs. 



This leads us to the question of the X-ray tube. 

 Its present efficiency is of the order of i/iooo. We 

 are led to inquire in what outstanding points has the 

 British tube made progress since Sir Herbert Jack- 

 son's introduction of the concave cathode in 1807 — 

 itself identical with one used by the late Sir William 

 Crookes some twenty years previously. Again, has 

 the British tube ever been superior to either the 

 German or American? We know the answer most 

 radiographers would give us. Before the war we 

 could^ not even make the glass for the bulb. But 

 that is another story. From the point of view of the 

 Old Countrv. it is a regrettable fact that it should 

 have been left to .America to develop (in the shape 

 of the Coolidge tube) pioneer research work done bv 



