102 Prof. Rutherford on Excited Radioactivity and 



of p was found to decrease with increase in the number of 

 alternations per second. With a constant speed of alterna- 

 tion the value of p increased with the voltage. 



When the value of p is small, the velocities of the carriers, 

 deduced from the equation, were found to be all too high, 

 and also inconsistent among themselves. There are several 

 disturbing factors which have a great influence on the value 

 of p when p is small. These factors are : — 



(1) Recombination and diffusion of the carriers. Unless 

 the electric field is strong, the carriers recombine and diffuse 

 to the electrodes. With a weak electric field the excited 

 radioactivity is distributed on both the positive and negative 

 electrodes. 



(2) Inequality of the electric field. In this simple theory we 

 have assumed that the potential-gradient between the plates 

 is uniform. This is far from being the case. The experi- 

 ments of Child and Zeleny have shown that there is always 

 a sudden drop of potential near the electrodes. The electric 

 field near the electrodes is consequently stronger than the 

 average. For this reason, wdien the carriers which reach 

 the plate B are only abstracted from within a short distance 

 of the plate, the value of p leads to too high a value of the 

 velocity. 



(3) Initial velocity of the carrier. From some considera- 

 tions which will be developed later (see § 9), it seems probable 

 that the positive charge on the carrier is due to the expulsion 

 of a negatively-charged particle of some kind from the 

 neutral molecule. The positive carrier may thus have 

 enough initial velocity imparted to it to carry it some 

 distance against the electric field. This will result in a 

 distribution of some excited activity on the anode, even with 

 a strong field. It is difficult to obtain direct experimental 

 evidence on this point, but there seems little doubt that such 

 an effect is present. 



In order to obtain consistent results, it was found necessary 

 to have a considerable difference between the strength of the 

 electric fields during the succeeding half alternations. If 

 the difference is small, the carriers take so long to reach the 

 plate A that recombination and diffusion of the ions become 

 important factors in determining the distribution of excited 

 activity. For the reasons we have explained above, it was 

 necessary to use fairly high voltages and correspondingly 

 rapid speed of alternation in the experiments. 



The following tables are examples of some of the results 

 obtained for different voltages and distances between the 

 plates. Temperature 18° C. Air fairly dry. 



