'664: Sir E. Rutherford and Prof. J. Barnes on Efficiency 



radiation were controlled by means of the " standardizing 

 vessel " described in a previous paper, and also by the current 

 passing through the Coolidge tube. This current was deter- 

 mined by the method described in a previous paper. The 

 deflexion of the galvanometer in the circuit was observed ; 

 the current through the tungsten spiral broken, and the 

 voltage kept at the same value by varying the speed of the 

 machine and by means of adjustable point discharge. The 

 deflexion observed under the latter conditions was due to the 

 current through the xylol resistances and voltage galvano- 

 meter in parallel with the Coolidge tube, and to electrical 

 losses in the leads or over the surface of the bulb. The 

 difference between the two readings served as a measure of 

 the actual current conveyed by the electrons from the heated 

 spiral. 



The earlier parts of the absorption curves in water for 

 64,000 aud 96,000 volts are shown in fig. 2. The absorption 

 is not exponential, but decreases steadily w T ith increase of 

 thickness of water. Knowing the initial saturation current 

 through the air with no absorber, the total ionization current 

 due to complete absorption of the radiation can be at once 

 deduced by determining the area included between the curve 

 and the two axes, assuming that 1 cm. of water is equi- 

 valent in absorbing power to S'2 metres of air at 15° C. 



Supposing that the X radiation from the tungsten anti- 

 cathode is emitted equally in all directions, the fraction of the 

 total radiation entering the ionization vessel was 8'6 x 10~ 5 . 



The intensity of the radiation was found to be directly 

 proportional to the current through the bulb. In the following 

 table the total ionization current in air due to complete 

 absorption of the whole radiation is expressed for each 

 voltage in terms of 100 divisions of the current galvanometer 

 which correspond to 2*92 x 10" 5 amp. 



Voltage. 



Current i 2 due 

 Cathode Rays. 



Total Ionization j 

 Current i x for : 

 Complete Absorp- X=« 1 /i 2 . /3. 



tion of the 



Radiation. 



X/fi\ 



48,000 

 64,000 

 96,000 



2-92 Xl0~ 5 amp. 



' 1 

 2oxl0-° amp. 'S6 '406 



4-2 „ „ 1-44 -459 



8-8 ,. „ 3-01 "540 



32 

 32 

 35 



Beatty (loc. cit.) concluded from his experiments that 

 X=*58 A/3 4 , where A is the atomic weight of radiator and j3 



