RONTGEN RAYS 301 



rays on the target or anticathode. This latter is placed at 45 

 to the beam of cathode rays, and is made of some metal of 

 high atomic weight, usually platinum or tantalum : as it tends 

 with a heavy discharge to get very hot, it is therefore often 

 water-cooled. An aluminium anode is joined outside the tube 

 to the anticathode; in the event of the discharge momentarily 

 reversing, the aluminium electrode rather than the platinum 

 takes on the function of temporary cathode, and there is, in 

 consequence, less sputtering of the metal and but slight black- 

 ening of the walls of the tube. 



As is well known, the pressure in a Rontgen-ray tube tends 

 to become lower with continued running of the discharge. 

 This "hardening" is attributed to absorption by the electrodes, 

 and, as Campbell Swinton has shown (1907), to the driving of 

 the gas into the walls. The effect is nowadays usually met 

 by sealing into the bulb a small platinum or palladium tube, 

 closed at its outer end; by the application of a flame to the end 

 of this tube a small quantity of hydrogen diffuses through the 

 hot metal, and the pressure in the bulb can be restored to 

 a convenient value. Another device for securing constancy of 

 pressure would be to join to the bulb a tube containing cocoa- 

 nut charcoal, which can be immersed in liquid air. By saturating 

 the charcoal with gas at the pressure desired, the vacuum can 

 be maintained unaltered for any length of time, no matter how 

 intense the discharge. 



Tantalum anticathodes do not appear to harden the tubes 

 so much as platinum ; they have too the advantage of a far 

 higher melting point (Pt 1750 C, Ta 2910 C). The radiation 

 from tantalum contains rather more soft rays, and about ten 

 per cent, less hard rays than the radiation from platinum under 

 the same conditions. 



The rays from an X-ray tube are not uniformly distributed 

 about the anticathode, as was formerly supposed. W. R. Ham, 

 at Chicago (1908), and H. Bordier, in Italy (1908), have recently 

 shown that there are large variations in the distribution. In 

 a plane determined by the beam of cathode rays and the normal 

 to the anticathode, the intensity is a maximum in a direction 

 at about 6o° from the normal, and falls off roughly by a cosine 

 law from this direction : in a plane at right angles to this, the 

 intensity diminishes much more slowly. 



It may be said that the intensity or quantity of the rays given 



