436 Mr. E. Rutherford on the Velocity and Mate 



for the surface ionization is probably, however, very small, 

 for the lower plate of the testing-vessel was of aluminium — a 

 metal which does not appreciably exhibit the phenomena of 

 surface ionization — and the upper of lead, in which the effect 

 is slight. In addition to this the plates were nearly 5 cms. 

 apart, so that the volume effect was very large compared with 

 that due to the surface. 



The velocities which have been determined are the sum of 

 the velocities of the positive and negative ions, but we have 

 so far given no direct experimental evidence to show whether 

 the velocities of the positive and negative ions are the same. 



In the case of air, an experiment which will now be de- 

 scribed seems to show that the velocities of the two ions are 

 equal or very approximately so. 



In previous determinations the calculation of the velocity 

 of the ions has depended on the truth of an equation which 

 has been experimentally verified as far as possible, but in 

 the case of air the velocity may be obtained by a method 

 not involving any theory depending on the rate of recom- 

 bination of the ions. 



Two large plane plates, A and B (fig. 3), were placed 

 parallel to one another, lb' cm. apart, on insulating blocks C 

 and D. The bulb was so arranged, in regard to the plates 

 A and B, that the radiation fell on the plate A and half of 



Fier. 3. 



A To Battery of 

 Cells 



the volume of air between A and B. No radiation reached 

 the air to the left of the dotted line E F in the figure, which 

 was 8 cm. from either plate. The plate A was connected 

 to one terminal of a large battery of storage -cells, the other 



