10 Prof, E. Rutherford on a Radio-active 



The results given in Table III. show that the rate of dimi- 

 nution of the number of ions is proportional to the number 

 present. 



Solving the equation, it is seen that 



log e (gr-Xn) = -X* + A, 



where A is a constant. 



When t = 0, n = 0; 



therefore A=log <1 <j. 



Thus 7/1 xn 



A, 



With a large potential-difference between the test-plates 

 the current i through the gas at any time is given by 



i = ne, 



where e is the charge on an ion. 



When a steady state is reached, — =0; and the maximum 



number N of ions produced per second by the radio-active 

 particles between the plates is given by 



and the maximum current I is given by 



Therefore i. — i _ -xt 



j-1 e . 



The current thus increases according to the same law 

 as a current of electricity rises in a circuit of constant 

 inductance. 



This result is confirmed by an experiment on the rise of 

 the current between two concentric cylinders. The thorium 

 oxide enclosed in paper was placed inside the cylinder. A 

 current of air was sent between the cylinders in order to 

 remove the emanation as rapidly as it was formed. The 

 current of air was then stopped and the current between the 

 two cylinders observed, by means of an electrometer, for 

 successive intervals after the current of air ceased. Table IV, 

 gives the results obtained. 



