Maximum Frequency of X Rays from a Coolidge Tube. 359 



A, = 1*277 X 10~ 8 cm.* must be excited for the appearance 

 of the " L " radiation, the theoretical voltage is 9600 volts, 



Table IV. 



Element. 



Atomic 

 number. 



\ for the 



Calculated 



Observed 

 voltage. 



Volts calc. 



/3 line. voltage. 



Volts obs. 



Aluminium . . . 



Chromium 



Iron 



Nickel 



Copper 



[Zinc 



13 



24 

 26 



28 



30 



7-91X10" 8 

 2-093 x 10 " 8 

 1-765 x 10" 8 

 1-506 X 10 " 8 

 1-402X10" 8 

 1-306x10""* 



volts. 

 1570 



5900 



7000 



8200 



8800 



9500 



volts. 

 1200 



7320 



9600 



10750 



11080 



11280 



13 

 •80 

 •73 

 •76 

 •79 

 •83 



while the voltage deduced from the equation v/v p = l — kV is 

 9800. The difference in this case between the observed and 

 computed voltage is about 2 per cent., and is not beyond the 

 experimental error. 



In order for the quantum theoiy to hold for excitation, it 

 is necessary that the whole of the energy of the electron 

 should be directly converted into energy of radiation. It 

 seems probable that if an electron makes an " end on " 

 collision with another, the whole of the energy of the one is 

 transferred to the other, but we have no definite evidence 

 that radiation is emitted in such a collision. Taking, for 

 example, the point of view proposed by Bohr t, that the 

 radiation arises from the fall of an electron from one ring of 

 electrons to the next, the essential antecedent to the emission 

 of radiation is the removal of an electron from one of the 

 rings. If the whole of the energy of the incident electron 

 is expended in ejecting the electron from the ring, it is to be 

 anticipated that the energy of the X radiation should equal 

 the energy of the incident electron. This condition should 

 approximately be fulfilled in the relatively sparse distribution 

 of electrons in the outer rings. In order, however, to excite 

 the higher frequencies, the electron must penetrate deep into 

 the atom where the electrons are more closely packed, and 

 part of its energy will be used up in setting neighbouring 

 electrons in motion, and only a fraction will be available to 

 eject an electron from its ring. Quite apart from any special 

 theory of the mechanism of radiation, such a factor must 



* See Barnes, infra, p. 368. 

 t Bohr, Phil. Mag. July 1913. 



