178 THE ELECTRIC ARC 



Still another method was given by Stark, Retschinsky, 

 and Schaposchnikoff. 1 They measured the drop in potential 

 near secondary electrodes placed in the arc, but such 

 measurements in the carbon arc are complicated by the 

 high temperatures of the exploring electrodes and by the 

 difference of potential which exists between such electrodes 

 and the surrounding gas, so that even these measurements 

 can not be considered as giving a definite proof of the 

 relative velocities of the negative and positive ions in the 

 carbon arc. 



When we turn to the mercury arc we have more definite 

 data. The measurements of Stark, Retschinsky, and 

 Schaposchnikoff were here made with cold exploring elec- 

 trodes and there would seem to be no question but that 

 they showed that the negative ions had the greater velocity. 

 The same thing was shown by Schenkel by means of the 

 Hall effect (p. 101), although the values given by him to 

 the velocity of the negative ions depended on certain 

 appearances which are not sufficiently definite to give 

 exact data. 



In addition to these measurements on the mercury arc 

 there is indirect evidence furnished by the very great 

 velocity of the negative ions in the discharge from hot 

 platinum and from hot lime in a vacuum and by the fact 

 that in the flame where the temperature is still much lower 

 than in the arc the velocity of the negative ions is greater 

 than the positive, so that in all probability in all forms 

 of the arc the velocity of the negative ions is much greater 

 than that of the positive. 2 



1 Ann. d. Phys., 18, 230; 1905. 



2 Certain theoretical considerations which will be published shortly in 

 the Physical Review lead to the same conclusion. 



