December, 1910. 



KNOWLEDGE. 



497 



It is an alternating current, hut the waves of one sign 

 preponderate over those of the opposite sign. Owing 

 to the fact that the resistance of an X-ray tube to an 

 inverse current is less than to a current in the right 

 direction, it is necessary that these lesser and inverse 

 waves be kept as small as possible — the reverse 



The newest coils are a very great advance on 

 anvthing of the kind that has been brought out 

 before. Not only are they very economical of 

 primarv current for the very heav\- secondary dis- 

 charge the\- give out, but they are for all practical 

 purposes quite free from inverse impulses. 



FiGfRE 6. 

 A radiograph showing a stone in the Kidney. 



current having a very deleterious effect on the life 

 and performance of the tube. 



The first attempts at making these " heavy 

 discharge " coils, as thev were called, were not very 

 successful ; in fact, until quite recently, all these 

 coils suffered from giving out too much inverse 

 current, which, as we have seen, is ver\- detrimental 

 to the tubes. 



A very good way to test a coil is to allow it to 

 discharge across an 8-inch air-gap, and take note of 

 the readings of the measuring instruments in both 

 primarv and secondary circuits. Under such con- 

 ditions a coil in the writer's possession gives a 

 current of about sixty milliamperes across the 

 air-gap, and without any means emploj'ed to 

 suppress any inverse current. The current in the 



