360 Maximum Frequency of X Rays from a Coolidge Tube. 



always enter into the energy of the electron finally available 

 to excite a characteristic radiation. 



This point of view offers a simple and probable explanation 

 of the reason why the quantum theory holds closely for 

 excitation of low frequencies by slow speed electrons, but 

 fails for high frequencies. The relation found experimentally 

 between vjv v for tungsten suggests that the correction for 

 this effect increases rapidly with the frequency. 



Summary. 



(1) The absorption curves in aluminium of the X radia- 

 tion from a Coolidge tube have been examined over a wide 

 range of constant voltages supplied by a Wimshurst machine. 

 While the main radiation is complex, the " end " radiation 

 is found to be absorbed exponentially. 



(2) The absorption curves for different voltages obtained 

 with an induction-coil are nearly the same as for the Wims- 

 hurst machine, and the penetrating power of the end radia- 

 tion is nearly the same. 



The maximum frequency of the " end " radiation for 

 voltages between 13,000 and 175,000 volts has been deduced 

 by examining its absorption in aluminium. The frequency 

 and penetrating power reach a maximum value at 145,000 

 volts, and are not altered by increase of the voltage to- 

 175,000. 



(3) The shortest wave-length emitted by the Coolidge 

 tube is 1*71 xlO -9 cm. or *17 A.U. The absorption coeffi- 

 cient of this radiation in aluminium is *39 (cm.) -1 , and 

 in lead 23 (cm.) -1 . The penetrating power of this radiation 

 is about 3/10 of the penetrating gamma rays from radium C. 



(1) The relation between the frequency and the voltage is 

 expressed by the formula /w = E — cE 2 , where E is the energy 

 of the electron, c a constant, and h Planck's fundamental 

 constant. This relation holds up to 142,000 volts when the 

 radiation has its maximum frequency. Evidence is given 

 that the quantum theory is directly applicable for the exci- 

 tation of waves of low frequency, but for higher frequencies 

 requires a correcting factor, the value of which increases 

 rapidly with the frequency. 



(5) A comparison is given of the radiation from a Coolidge 

 tube with the gamma radiations emitted by radium B and 

 radium C. 



University of Manchester, 

 July 1915. 



