230 



SCIENCE-GOSSIP. 



mind as to ts working, the writer would strongly 

 advise him for safety sake, to first insert a resistance 

 which may be gradually cut down as required, other- 

 wise serious consequences to the coil may accrue. 

 Certainly for a large coil, say one giving a 20-inch 

 spark, not more than 15 amperes should be sent 

 through, and of course a smaller current for smaller 

 coil. 



Fig. 20. X-Ray Tube. 



Fluoroscopes. 



When examining a patient by means of the 

 fluorescent screen (ante, p. 135), it is sometimes 

 found very inconvenient to shield the screen from 

 external light, such as that from the windows or even 

 from the spark made at the break belonging to the 

 coil. Instruments have been devised to overcome 

 this difficulty and are known by a variety of names 

 such as fluoroscopes, radioscopes, kryptoscopes, etc. 

 Such a one is shown in fig. 18. It 

 consists of a camera bellows fitted to a 

 frame, in one end of which the fluores- 

 cent screen is made to slide. The front 

 end of the fluoroscope is fitted with 

 a fur-lined aperture to receive the ob- 

 server's eyes. The arrangement is port- 

 able folding up into a small compass, as 

 is seen in fig. 19. These fluoroscopes 

 are a boon to radiographers, as the conditions of 

 various parts of the body may be viewed easily in the 

 daytime. 



The X-Ray Tube. 



We come now to discuss the last, but certainly not 

 the least important accessory to an X-Ray outfit — the 

 X-Ray vacuum tube. Except in detail, the form of 

 the tube now in general use does not differ from the 

 original pattern of Mr. Jackson, of King's College. 

 This, as is seen from fig. 20, consists of a concave 

 aluminium cathode, and a piece of sheet platinum as 



takes place through the tube, and if the aluminium 

 cup is made the cathode, the cathode rays emanating 

 from the cathode impinge upon the platinum anode 

 or anti-cathode, as it is called, and are scattered out 

 through the glass as Rontgen rays. In a later form 

 ot tube there is an aluminium anode in addition to 

 the anti-cathode, but it is the latter that receives the 

 bombardment of the cathode rays. When used, this 

 form of tube has. the anode and anti-cathode con- 

 nected together outside, so that both are in com- 

 munication with the positive terminal. 



When w : orking with these forms of X-Ray tubes, 

 two difficulties present themselves. Firstly : under 

 different conditions of working and different spark 

 lengths, the one tube with its one degree of exhaus- 

 tion and one value of resistance cannot be adapted. 

 Secondly : upon continued working, it is found that 

 the 'exhaustion, and therefore the penetrating value, 

 of the tube increases, so that finally, in spite ot 

 repeated heatings by a Bunsen flame or other source, 

 to increase the pressure inside, the resistance of the 

 tube becomes so high that the electrical discharge 

 will not take place under the same conditions for 

 which the tube was originally selected. 



Fig. 22. Special X-Ray Tube for Strong Discharges 



anode, supported about the middle of the glass bulb 

 and inclined at an angle of about 45 to the line 

 joining the anode and cathode. The tube is ex- 

 hausted very highly, otherwise no X-Rays will be 

 produced, yet not too highly to prevent electrical 

 discharges passing through at all. When a discharge 



MG. 21. Adjustable Cathode X-Ray Tube. 



These difficulties have been overcome, chiefly 

 owing to the work of Mr. A. A. C. Swinton, whose 

 results upon the modus operandi in the interior of the 

 tube, also upon the conditions affecting the emission 

 of X-Rays, have proved of very great .importance in 

 the work. Among other things Mr. Swinton found, 

 with experimental tubes made in his laboratory, that 

 if the anode of the tube be so arranged that the 

 distance between it and the cathode could be ad- 

 justed, then a ready and very simple means was at 

 hand whereby the resistance and penetration could be 

 altered to suit the varied conditions 

 imposed. The nearer the anode is 

 placed to the cathode the higher the 

 resistance, and consequently the higher 

 the penetration of the tube, and vice 

 versa. In moving the anode of a tube, 

 however, the point of origin of the 

 X-Rays is also moved for each adjust- 

 ment, which is certainly a disadvantage, 

 especially when a difficult radiograph, 

 requiring a lengthy exposure, is being 

 taken. While, therefore, taking advantage of Mr. 

 Swinton's very useful principle of varying the dis- 

 tance between anode and cathode, Dr. Dawson 

 Turner, in conjunction with the writer, reversed the 

 arrangement by making the cathode movable and 

 keeping the anode fixed. They added a further 



