produced by the Action of Thorium Compounds. 179 



plate between two test-plates 4 cms. apart was 1*7 x lO -11 

 amperes. 



Fig. 8 shows the results graphically. From the table it 

 will be seen that the intensity has reached half its final value 

 in about twelve hours. 



Fig. 8. 



10 



90 

 80 

 70 

 60 

 50 

 40 

 30 

 ^0 

 10 



10 20 30 40 50 60 70 80 90 100 



We will now consider the conditions which influence the 

 increase of the intensity of radiation from a given surface 

 exposed to the action of a thorium compound. We will 

 suppose that the surface to be made radio-active is negatively 

 charged. 



Two opposing actions are evidently at work. Fresh radio- 

 active particles are being continually carried to the plate, 

 while the intensity of the radiation given out by the active 

 surface continually diminishes, owing to the radiation of 

 energy. A steady state will be reached when the rate of 

 increase of intensity due to the supply of fresh radio-active 

 particles is equal to the rate of decrease of the intensity due 

 to the radiation of energy from the active surface. 



Let I be the intensity of the radiation at the surface of the 

 plate at any time. The rate of diminution of the intensity is 

 equal to LI, since the intensity I at any time is given by 





















| — O | 























1 



/ 



















1 



/ 





















/ 



















/ 



/ 



















/ 





















/ 





















/ 





















/ 





/ 



''/MB 



/A/ h 



'oi//?s 











and 



I = I e- L ' 



dl 



dt 



= -LL 



N2 



