Mobility of the Negative Ion. 



449 



positive ion at 200 mm. pressure. The mobilities are given 

 in arbitrary units, the mobility of the negative ion at 

 atmospheric pressure being equal to 0*72 in these units. 

 The Table II., corresponding to curve I. ; is given to show 

 the procedure of the experiments. 



Table II. 







p = 748 



mm. 



, d=10 



mm. 







V. 



c. 



P. 



c 

 P" 



v. 



c. 



P. 



c 

 P ' 



80 



10 



14 



71 



2000 



52 



60 



•88 



200 



32 



44 



•73 



2120 



24 



21 



114 



600 



4G 



63 



73 



2200 



48 



39 



1-23 



1000 



38 



53 



■72 



2400 



71 



47 



1-51 



1400 



51 



69 



74 



! 2600 



65 



34 



1-91 



1600 



26 



36 



72 



3000 



89 



40 



2-23 



1720 



31 



43 



73 



3400 



76 



30 



253 



1800 



35 



46 



76 



4000 



92 



34 



2-77 



1880 



43 



54 



80 











The curves show that when the critical value of the electric 

 force is reached the mobility of the negative ion begins to 

 increase, at first rapidly. At atmospheric pressure the 

 mobility is doubled when the electric force is increased from 



1800 to 2400 - - ; at 400 mm. pressure the mobilities 



cm. 



at 450 and 2000 



volt 



cm. 



are as 1 to 5. With larger forces 



the mobility increases less rapidly, and it may be noticed from 

 the curves that it apparently tends to reach a constant value. 

 The curves show also that at high pressures the velocity of 



the negative ion cannot be expressed as a function of — . 



The determination of the critical electric force X x at a 

 given pressure involves great difficulties. At high pressures 

 the result of experiments carried out under the same 

 conditions varies from day to day to the extent of 20 per cent. 

 The amount of moisture present in air does not apparently 

 affect the results, although in these experiments the air was 

 never dried with special care. The effect may be due to 



