the X Radiation from a Coolidge Tube. 155 



It will bo seen from the above table that the thickness of lead 

 through which the radiation was measurable increased with the 

 voltage applied. This is a result not only of the increase of 

 the penetrating power of the radiation, but also of the large 

 increase with voltage of the intensity of the radiation. With 

 a voltage of 196,000, the radiation was detected and measured 

 through 10 mm. of lead. In this case, the intensity of the 

 radiation after passing through this thickness of lead was 

 considerably less than one millionth of its initial value. No 

 doubt by the use of still more powerful rays and more sen- 

 sitive methods of measurement, the radiation could be detected 

 through a still greater thickness. The maximum voltage 

 applied (196,000 volts) was about the limit of capacity of 

 the induction-coil under the working conditions. In addition, 

 I should adjudge this voltage to be about the limit of safety 

 for the bulb itself, so that no attempt was made to examine 

 the penetrating power of the radiation for still higher 

 voltages. 



Certain interesting points arise in considering the results 

 given in the table: — 



(1) There is not much change in the value of ,a for the 

 end radiations between 79,000 and 144,000 volts, and no 

 observable change in fju between 105,000 and 144,000 volts. 



(2) Between 105,000 and 144,000 volts the radiation is 

 absorbed nearly exponentially with a value of fju—22. 

 Above 144,000 volts the absorption is no longer exponential, 

 but the value of fju decreases progressively with increase of 

 thickness of absorber. This is best shown by the results for 

 183,000 volts, in which the value of jll decreases from 26 to 

 12 as the thickness of absorber is increased from '7 to 

 70 mm. 



These results, which are at first sight peculiar and un- 

 expected, can be very readily explained by taking into 

 account the absorption of rays of different frequency by lead. 

 In a recent paper*, Hull and Miss Rice have carefully 

 examined the absorption coefficient of lead for X rays of 

 different wave-lengths, obtained by reflexion from a rock- 

 salt crystal. For wave-lengths greater than 0*149 A.U., the 

 absorption in lead obeys the law /m/p = 430 X, 3 + 0*12, where \ 

 is the wave-length in Angstrom units and 0*12 is the 

 assumed mass- scattering coefficient, a/ p. The value of fju/p 

 suddenly increases for values of \ below 0*149 A.U. owing 

 to the presence of a characteristic absorption-band in lead. 

 The presence of this sharp absorption-band has been shown 

 iilso photographically by Hull and Miss Rice and by 



* Hull and Miss Eice, Phys. Kev. viii. p. 326 (1916). 

 M2 



