McClkklan]) and Kennedy — The Large Ions in the Atmosphere. 73 



Description of Apparatus. 



To obtain saturation currents when dealing with ions of very small mobility, 

 it is necessary that the gas should either be drawn through a very intense 

 electric field, or else that its time of passage through the field should be 

 prolonged. To allow of the use of moderate potentials, and in order to be 

 able to deal, as we wished, with large volumes of air, the apparatus took the 

 following form. The air was drawn by means of a motor^and fan through a 



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long, wooden, rectangular tube about 550 cms. in length. The internal cross- 

 section of the tube is shown in the diagram, fig. 1. It was 27 cms. in width 

 and 4'5 cms. deep. The top and bottom inside surfaces were covered with 

 tinfoil [D) laid on a thin sheet of paraffin, the insulation being sufficiently 

 good to enable the tinfoil surfaces to be kept at the desired potential by 

 means of small storage-cells. Half way between these horizontal conducting 

 surfaces there was placed a glass plate [A) covered with tinfoil. The plate 

 was supported as shown in fig. 2 on a few small metal rods {B) projecting 

 through the sides of the tube, and was insulated from the rods by paraffin 

 wax(i?), the rods themselves being connected to earth. The top and bottom 

 conducting-surfaces {D) were kept at a high voltage, and the middle plate (^) 

 was joined to a Dolezalek electrometer by a suitably shielded wire. From 

 the method of support just described, no leak to the electrometer system 

 could occur except through the air. The middle plate and the tinfoil 

 surfaces on the top and bottom of the tube were 457 cms. in length. 



