456 



Mr. S. Ratner on the 



In the curves ( + ) and (— ) (fig. 7) the wind-pressure is 

 plotted against the electric force at 450 mm. pressure, in 

 the case when the ions moving towards the grating are 

 positive and negative respectively. These curves are in full 



Fkr. 7. 



~2 



200 400 600 800 1000 I2Q0 1400 



Electr.c ^orce in V°1L S 



F,g 7 



1600 tSOO 2000 



agreement with the results relating to the mobility of the 

 negative ion given above. For small forces the wind- 

 pressure is small, as the ions in this case recombine on their 

 way and traverse only a small part of the distance between 

 the plates. The wind-pressure increases with the force 

 until the saturation current is reached, and with further 

 increase of the force a striking difference between the two 

 curves is observed. Under these conditions the mobility of 



the negative ions, as measured by the ratio p , begins rapidly 



to increase, and since c is constant in these experiments, the 

 wind-pressure P produced by them must decrease at the same 

 rate ; while the mobility of the positive ions, and conse- 

 quently the wind-pressure due to them, is not affected by the 

 increase of the force. When the plate A is negatively 

 charged and the negative ions are moving towards the 

 grating, the wind-pressure produced by them diminishes 



