348 Sir E. Rutherford, Prof. Barnes, and Mr. Richardson : 



penetrating with increase of thickness of absorber *. Finally, 

 for the higher voltages, when the current is reduced to about 

 1/500 of its value, the absorption curve becomes very nearly 

 a straight line until the radiation is completely absorbed. 

 This shows that the end radiation is approximately homo- 

 geneous, and it is the absorption coefficient of this end 

 radiation that was carefully determined. 



From 40,000 volts upwards, the end radiation was ab- 

 sorbed exponentially over a considerable range of intensity, 

 but below this voltage the penetrating power of the radia- 

 tion appeared to slightly increase until it was completely 

 absorbed. The analysis of the radiation by reflexion of 

 crystals, as given in the following paper, p. 361, shows that 

 the il L " characteristic radiation of tungsten escapes from 

 the bulb. Its intensity, however, is greatly reduced by 

 passing through the glass of the bulb, which was found 

 by direct measurement to be about '5 mm. thick. Careful 

 observations were made of the voltage for which the ioniza- 

 tion in the testing-vessel was first measurable. In order to 

 increase the electrical effect, the testing-vessel was rilled 

 with sulphur dioxide. No ionization was observed below 

 10,300 volts, and it then increased very rapidly with the 

 voltage. The absorption of the main radiation by aluminium 

 was examined at the lowest possible voltage, and was found 

 to be fi=69 cm. -1 in aluminium. This coefficient no doubt 

 corresponds to that of the characteristic u L ,J radiation of 

 tungsten under experimental conditions. The absorption 

 coefficient of this radiation measured by Chapman f was 

 found to be 81, but as in passing through the bulb the softer 

 components were relatively cut out, the issuing radiation 

 would be expected to be more penetrating than that observed 

 under normal conditions with no absorber. The range over 

 which the ionization could be measured increased rapidly 

 with the voltage above 10,000 volts, but on account of the 

 presence of a large proportion of softer radiations, it was 

 difficult to determine with certainty the absorption coefficient 

 of the end radiation for the lower voltages. In every experi- 

 ment it was found, however, that the penetrating, power of 

 the radiation increased rapidly and regularly with increase- 

 of the voltage. 



* Preliminary measurements on the absorption of the radiation from 

 a Coolidge tube have been made by S. Russ (Journ. Rontgen Soc. April 

 1915), using an induction-coil to excite the rays. He observed that the 

 radiations were not homogeneous, but tended -to become so with increase 

 of thickness of absorber. 



t Proc. Roy. Soc. A. lxxxvi. p. 439 (1912). 



