495 



KNOWLEDGE. 



December, 1910. 



tube, and so arranged that when the resistance of 

 the main tube rises hevond a certain degree the 



FlGlRK 4. 



Taking a stereoscopic radiograph of the Knee Joint. 



The pl.^te is passed under the knee Ity iiie.'ins of a tray sliding in the shallow liox helow. The Xray tube in the black box 

 below the conch is onl)- used when a canvas lop is substituted for the one here in use. 



electric current passes through the small bulb, heat- 

 ing the porous material and causing it to give off 

 some of its occluded gas. 

 This passes into the large 

 tube, lowers its resistance, 

 and the current resumes its 

 proper course. 



Apart from the presence 

 of the vacuum regulator and 

 the larger size that is now 

 generally adopted, the 

 modern X-ray tube does 

 not differ in an}- essential 

 particular from the original 

 one designed and made b\- 

 Professor H. Jackson. It 

 is a pleasing duty to record 

 the fact that he placed his 

 important discoverv freeh' 

 at the disposal of anyone 

 who wished to make use of 

 it, and has not derived anv 

 more tangible benefit there- 

 from than the gratitude of 

 X-ray workers throughout 

 the world. 



We must now turn to the 

 improvements made during 

 recent years in the coils and 

 other high potential trans- 

 formers. Before the days of the X-ravs induction 

 coils were rated or comjjared with one another 



according to their spark length. The maker of a 

 big coil aimed at a certain spark length, and was 



not ^■er\■ particular as to 

 how it w as olitained : but 

 the curious fact was. that 

 when one of these big coils 

 was used for X-ra\- work 

 the results obtained were 

 not alwa\s those expected, 

 though it was generally 

 agreed that large coils gave 

 better results than small 

 ones. About the beginning 

 of the present centurv many 

 coils were in use giving 

 sparks of twenty, thirty or 

 more inches in length : but 

 as it was realised that a 

 sfjark length of anything up 

 to six or eight inches \\ as 

 sufficient to overcome the 

 resistance of an\- usefvd 

 X-ra\- tube, it became 

 apparent that nothing was 

 to be gained hv increasing 

 the sparking distance of the 

 coil be\ond a certain point, 

 which has been found to be 

 about fifteen inches. The 

 only other thing to do was to 

 increase the quantity of current available, and so 

 drive a larger volume of current through the X-ray 



i-lGUKi:: 5. 

 Radiography of the Kidney. 



The metal cylinder upon which the X-ray tube is mounted is pressed down into the abdomen, reducing its thickness and 

 preventing all movement of the kidney itself. 



tube. As is well known, the current ivom any induc- 

 tion coil is not continuous, nor even unidirectional. 



