15 



It appears from the curves that it is the strength of field 

 in the immediate neighbourhood of tlie gauze and not so 

 much the actual difference of potential between the inner and 

 outer cylinders that determines the number of ions received 

 by the central electrode. As in the experiments so far de- 

 scribed, this field was by no means uniform, it was decided 

 to re-design the apparatus so as to provide for this condition. 



An apparatus as shown in fig. 4 was constructed, hav- 

 ing three gauzes set parallel to each other with their planes 

 at right angles to the axis of the tube. The central gauze, 

 to which the electrometer was connected, consisted of a brass 

 gauze with 20 strands to the inch, each wire being '01 in. in 



diameter, soldered to a thin metal ring and carefully at- 

 tached to a ring of sulphur in such a way as to allow the 

 air to pass only through the gauze. Earthed metal rings, as 

 shown in the diagram, were found necessary to protect the 

 sulphur from the direct influence of the high-pot-ential plates. 

 On each side of this middle gauze were arranged gauzes which 

 could be easily replaced, and so that their distance from the 

 middle gauze could be varied. In the first set of experiments 

 these gauzes were of brass with 40 strands to the inch, the 

 distance between the top and middle gauzes being 6'5 mm., 

 that between the bottom and middle gauzes 7 5 mm. 



In fig. 5, curve (.4 — ), is shown the effect of varying the 

 potential applied to the top gauze, always, however, keeping 



