362 Sir E. Rutherford and Prof. J. Barnes on Efficiency 



where /3 is the velocity of the cathode as a fraction of the 

 velocity of light. From the curves given by Beatty, it 

 would appear that the maximum speed of the cathode rays 

 employed by him corresponded to 23,000 volts. 



In the experiments with the Coolidge tube we have exa- 

 mined the efficiency of production of the radiation escaping 

 from the bulb for voltages 48,000, 64,000, and 96,000 volts. 

 The rays were excited by a Wimshurst machine, and the 

 current through the bulb measured by a galvanometer. The 

 general arrangements for controlling and calibrating the 

 voltage were the same as those described in the accom- 

 panying paper. For the high voltages employed, the 

 radiation is too penetrating for complete absorption in a 

 reasonable length of air or other gas. In order to overcome 

 this difficulty, we have measured the ionization due to a 

 definite length of a beam of X rays in air, and deduced the 

 absorption in air indirectly by determining the absorption 

 curve of the radiation in water. It is known that the 

 absorption of X rays by complex molecules is additive. The 

 absorption of the radiation by water is mainly due to the 

 oxygen atoms, whose atomic weight differs only slightly 

 from that of the average atomic weight of air. For the 

 relatively penetrating rays employed, we can assume with 

 very little error that the absorption of the X rays by water 

 is very nearly equal to that of a column of air of the same 

 thickness compressed to the same density. The absorption 

 by 1 cm. thickness of water is thus equivalent to that by 

 8'2 metres of air at laboratory temperature. 



The ionization due to the X rays was measured in air 

 without the X rays impinging on the electrodes in order to 

 avoid the introduction of surface effects due to scattering or 



Fig. 1. 



tery 





T~ 



UT 



V, 



electrometer 



^iE&rth 



excitation of characteristic radiations. The general arrange- 

 ment of the apparatus is shown in fig. J. The ionization 

 vessel consisted of a rectangular box (20 x 12 x 12 cm.) lined 



