20 



it negative, while maintaining the bottom gauze at a nega- 

 tive potential of 41 volts. A draught of 10 cms. per second 

 was maintained as in the earlier experiments. It will be 

 seen from the curve, that with 50 volts applied to the top 

 gauze, a much larger current passes to the middle electrode 

 than when the top gauze is connected to earth ; but that fur- 

 ther increases in the potential of the top gauze cause very 

 little further increase in the current. Curve (A + ), in the 

 same figure, shows how the current varies when the bottom 

 gauze is still maintained at a potential of —41 volts, but 

 with the potential of the top gauze varied through a range 

 of positive values. With the bottom gauze at —41 volts, and 

 the top gauze at + 500, practically no current passed to the 

 middle electrode — that is to say, a large number of negative 

 ions which would otherwise be drawn to the middle elec- 

 trode are now completely dragged through it. Similar re- 

 sults occurred when the bottom gauze was maintained at a 

 positive potential of 41 volts, the greatest diminution in the 

 current to the middle gauze being observed when the top 

 gauze was made negative. 



To prevent any interaction of the two fields above and 

 below the middle screen, the gauze was replaced by a screen 

 consisting of two gauzes, similar to the first, set parallel to 

 each other and at a distance apart equal to 2 mm. Curves 

 (B — ) and {B + ), fig. 5, were then obtained by following out 

 the procedure described in the previous test, from which the 

 curves (A -) and (-4+) were derived. The same type of re- 

 sult is obtained, but greatly reduced. Single gauzes with 

 finer mesh produced much the same eifect as the double gauze. 



It appears from these results that when the wider single 

 gauze is used for the central electrode, some of the ions which 

 it should apparently stop are blown completely through it, 

 and more the wider the gauze ; but that these are collected 

 by the gauze when the field in the top chamber drives them 

 with sufficient force against the draught. If, then, the top 

 gauze be of the same sign as the bottom gauze, any field 

 in the top chamber greater than about 100 volts per cm. 

 ensures practically all the ions being caught by the middle 

 electrode. In subsequent experiments the top chamber was 

 always so arranged, and the screen of two gauzes was used as 

 the central electrode. 



The strength of field in the lower chamber was varied 

 and corresponding currents measured. Curves were obtained 

 for depths of bottom chamber, 75 mm., 11*5 mm., and 235 

 mm., respectively. The capacity was determined by an auxil- 

 iary condenser in each case, and after correcting the read- 



