710 Prof. E. Rutherford and Mr. S. J. Allen : Excited 



Since 1= ^ I when t = A5 mins. 



\=-00026. 

 If the excited activity produced on the wire is due to a 

 uniform rate of deposit of radioactive material the radiation 

 from which decays with the time according to the above 

 equation, it necessarily follows* that the intensity I after a 

 time of exposure t is given by 



I=I„(l-e-«), 



where I is the maximum value of the intensity reached after 

 -a very long exposure. 



If this result is correct the amount of excited activity in a 

 oiven wire for a fixed voltage should reach half its final 

 value in 45 minutes. 



Some experiments have been made on this point with wires 

 exposed in the open air for different times. The amount of 

 excited radioactivity in the air was found, however, to be too 

 variable to test the truth of the equation. The results, how- 

 ever, showed that the amount of activity increased at first 

 roughly in proportion to the time, but after three or four hours' 

 exposure reached a practical maximum. More accurate ex- 

 periments on this point are at present in progress, using a 

 closed room instead of the open air, when the amount of 

 excited activity is much more constant. 



The amount of excited radioactivity from the air increased 

 with the voltage of the exposed wire. On account of the 

 variation of the amount of excited radioactivity in the air 

 from hour to hour and day to day no definite results on the 

 variation of the amount of excited radioactivity with the 

 voltage were obtained. 



Effect, of Weather Conditions. 



A large number of experiments were made on the effect of 

 atmospheric conditions on the amount of excited radioactivitv 

 from the air. The wire was usually exposed for 30 minutes 

 at a voltage of —25000 volts, outside a laboratory window, 

 at a height of about 15 feet from the ground. The results 

 showed that the amount of excited activity produced from 

 the air varied very greatly with the atmospheric conditions. 



Other conditions being the same, a bright clear day gave 

 more excited activity than a dull cloudy day. The effect of 

 temperature was not very marked. If anything slightly 

 more activity was obtained on a bright day during the 

 Canadian winter, with a temperature of about —20° C, than 

 * E. Rutherford, Phil. Mag. Feb. 1900, p. 180. 



