176 Prof. E. Rutherford on Radioactivity 



made active. In order to test the rate of decay of the 

 intensity, the active rod was placed inside a cylinder and 

 concentric with it. The current between the two cylinders 

 for a potential-difference of 50 volts was measured in the 

 usual manner, and at intervals of several hours. 



Length of rod = 31*5 cms. 



Diameter = *40 cms. 



Testing-cylinder, inside diameter = 7'3 cms. 



Time in hours. 



C urrent. 







1 



7-9 



-640 



11-8 



•474 



234 



•196 



29-2 



•138 



32-6 



•103 



49-2 



•0370 



62-1 



•0186 



71-4 



•0086 



The value of the maximum current, which is taken as 

 unity, was 1*6 x 10" u amperes. 



Fig. 7 shows graphically the results obtained. The results 

 show that the current through the gas (which is proportional 

 to the intensity of the radiation) diminishes in geometrical 

 progression with the time. The time taken for the intensity 

 of the radiation to fall to half its value is about eleven hours. 

 Tf I be the intensity at the beginning, the intensity I 

 after a time t is given by 



T_T p -Lt 



where L is a constant. 



The above law appears to hold accurately for all substances 

 made radio-active. No difference in the rate of decay has 

 been observed, whether the radiation is on a plate of large 

 area or concentrated on a fine wire. The rate of decay is 

 also independent of the substance made radio-active. A piece 

 of paper, mica, or metal,, all give the same rate of loss of 

 intensity. As far as experiments have gone, the rate of 

 decay is unaffected by the pressure of the gas surrounding it, 

 or whether the air is dry or full of moisture. The same rate 

 of decay has always been obtained under all the conditions 

 tried, provided the surface is not acted on mechanically or by 

 chemicals. 



The mean value of L deduced from the above results is 



L = -0000189, 



