142 



MK. J. S. TOWNSEND ON THE DIFFUSION OF IONS INTO 



connecting it to a terminal of a battery of 40 lead cells, the other terminal of which 

 was connected to earth. The electrode, E, was joined to a pair of quadrants of an 

 electrometer, the other quadrants and the case being connected to earth. 



In order to obtain a uniform stream of air, the tube, G, was connected to a 

 gasometer, and the velocity of the gas along the tubes, T, could be calculated by 

 observing the rate at which the cylinder of the gasometer fell. When the bulb is 

 giving out rays, the gas, as it passes the aluminium window, becomes a con- 

 ductor, and the ions are carried with the stream into the tubes T,. In passing 

 through these tubes some of the ions are discharged by the sides, and the rest on 

 coming into the field of force (caused by the difference of potential of 80 volts 

 between the electrode and the tube C) are removed from the gas. It will be seen 

 that no external force acts on the ions until they escape from the tubes T,, since all 

 parts of the apparatus, except E, are in metallic connection with the large tube A. 



Fig. 2. 



W, m , I a 4 T, JL! <s^ 



When the potential of A is positive, the positively charged ions are collected on 

 the electrode, and the deflection of the electrometer needle is proportional to the 

 number of these ions, which come through the small tubes. The negative ions are 

 collected on the electrode by making the potential of A negative. 



If the motion of the gas past the electrode were steady, it would only require a 

 difference of potential of a few volts between the electrode and the tube in order 

 to remove all the ions from the gas. This, however, is not the case, since the motion 

 of the gas as it escapes from the tubes, T, is turbulent, so that it is necessary to use 

 a large force in order to get the maximum deflection on the electrometer scale. It 

 was found that when the potential difference was changed from 80 to 40 volts, that 

 the deflections were not appreciably altered ; any voltage, therefore, between 40 ami 

 80 would suffice to remove all the ions. 



