714 Prof. E. Rutherford and Mr. 8. J. Allen : Kecked 



There is strong evidence that a wire charged to a high poten- 

 tial attracts the carriers over a large volume of air. It was 

 experimentally found that the amount of excited activity 

 obtained from a wire charged to - 20,000 A'olts in a cylin- 

 drical vessel of volume 141,000 cub. cms., when outside air 

 was drawn through it at a rate of 500 cms. per sec, was only 

 =a few per cent, of the amount obtained from the same wire 

 in the open air. 



The increase of excited activity observed on days on which 

 a strong wind is blowing is, on this view, due to the con- 

 tinued supply of fresh carriers which are brought in the 

 sphere of action of the electric field. Since the exposed wire 

 merely acts as a collector of radioactive carriers under the 

 influence of the electric field the amount and nature of the 

 excited radiation should be independent of the nature of the 

 conductor, and this is found to be the case. 



It thus appears probable that radioactive carriers are con- 

 tinually produced from some constituent of the atmosphere. 

 but at a rate depending on atmospheric conditions. Bright 

 •clear weather appears to be the most favourable condition. 



Since the earth is nearly always charged negatively with 

 regard to the upper atmosphere, it follows that these radio- 

 active carriers are being continually deposited over the 

 surface of the earth. We must thus regard the earth as 

 covered with an invisible layer of intense radioactive material 

 which ionizes the air strongly within a few centimetres of 

 the surface. The presence of these carriers in the volume of 

 the air will also cause the production of fresh ions throughout 

 the atmosphere, for each carrier acts as a radiating centre. 

 A hill or mountain peak, or any high mass of rock or 

 land, concentrates the earth's electric field upon itself and 

 consequently it will receive more excited radioactivity per 

 unit area than the level plain. Elster and Geitel have pointed 

 out that the greater ionization observed in the neighbourhood 

 of projecting peaks, receives a satisfactory explanation on this 

 view. 



Spontaneous Ionization of the Air. 



The experimental arrangement shown in fig. 5 was 

 employed for determining the number of ions produced 

 per c.c. per sec. in air and the variation of the ionization 

 current with the strength of the electric field. 



The current was observed by means of the electrometer 

 between two concentric zinc cylinders A and B, 154 ems. in 

 length. 25*5 and 7'5 cms. in internal diameter. The cylinders 

 were placed vertical and the base of both cylinders closed. 



