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



first place, no correction has been made for the disappearance 

 of ions by diffusion to the sides of the vessel. This can be 

 shown to be quite an important factor in causing a low 



value of — . 



Curve II. (fig. 6) shows what an important influence the 

 presence of dust has on the shape of the current voltage 

 curves. In addition, it has been assumed, for simplicity of 

 calculation, that the potential gradient is not disturbed by the 

 movement of the ions. Experiment and theory have, how- 

 ever, shown that there is a sudden drop of potential near both 

 electrodes and that the electric field some distance from them 

 is less than if no ions were present. All of these three causes 

 act in the same direction and tend to give too low a value 



. N 

 of — . The agreement between theory and experiment is thus 



as close as could be expected under the experimental 

 conditions. 



The results show clearly that, when air is kept in a closed 

 vessel and no electric field is applied, the number of ions per 

 unit volume, when equilibrium occurs between the rate of 

 production and dissipation, is more than 50 times the number 

 produced per sec. per unit volume. 



Velocity of the Ions. 



Some experiments were made to obtain an approximate 

 estimate of the velocity of the ions which are spontaneously 

 produced in air and at the same time to determine the 

 number of ions per unit volume present in the outside air. 



For this purpose, the apparatus shown in fig. 7 was 

 employed. Air from the outside of the building was drawn 

 through a zinc cylinder, length 200 cms., diameter 30 cms.. 

 by means of a fan driven by a motor. 



The air in its passage through the tube passed through 

 three circular parallel wire gauzes, A, B, C, 2 cms. apart and 

 insulated from each other. The first gauze A was conneeted 

 to earth, the second B to the electrometer, and the third C to 

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

 of which was to earth. A guard-ring, connected to earth. 

 was arranged between B and C to ensure there was no 

 conduction leakage across the insulators between B and C. 



Suppose gauze C is charged positive. The positive ions, 

 carried with the current of air between the gauzes, start to 

 travel up against the current of air, while the negative ions 

 travel to the positive electrode with the current. If the 

 velocity of the positive ions in the electric field is greater 



