THE MERCURY ARC 



101 



the arc, although it did not increase as rapidly as the pres- 

 sure. The following is an abridgement of the table given 

 by them. 



Kuch and Retschinsky also examined the temperature 

 in quartz lamps using a thermojunction. 1 They found 

 that at the middle of the tube the temperature became as 

 high as 1710 C., which was the highest that their thermo- 

 junction would register. The temperature at the sides was 

 at all times lower than that in the middle. 



Velocity of the Ions in the Mercury Arc. The 

 velocity of the negative ions in the mercury arc is much 

 greater than that of the positive ones. 2 This has been 

 shown by placing two secondary electrodes in the con- 

 ducting vapor of this arc and passing a current between 

 them. On examining the potential between these elec- 

 trodes, it is found that the drop in potential near the posi- 

 tive is many times greater than that near the negative. 

 As will be shown in the discussion of the theory of electrical 

 discharge, this shows that the negative ions have the 

 greater velocity. 



The same thing is shown by the work of Schenkel 3 on the 



1 Ann. d. Phys., 22, 595; 1907. 



2 Stark, Retschinsky and Schaposchnikoff, Ann. d. Phys., 18, 230; 1905. 



3 Mitt. d. Phys. Ges. Zurich, p. 13, 1906. 



