THE MERCURY ARC 91 



at the bottom of the filament, which quickly extends the 

 whole length of the tube, since the resistance of the carbon 

 filament is much greater than that of the mercury vapor 

 after the arc is started. 



FIG. 27. 



Temperature of the Mercury Arc. As has been stated 

 the mercury arc affords better opportunities than other 

 arcs for the accurate measurements of those quantities 

 which are important in forming a theory of the discharge. 

 While this is true, there are still several of these about which 

 we are more or less ignorant. Either the different ob- 

 servers have recorded different results, or else some valid 

 criticism may be made on the methods used. 



