ARC IN AIR BETWEEN OTHER SUBSTANCES 67 



the cathode drop is approximately 13 volts. Modifica- 

 tions of these were also found where the cathode drop is 

 slightly less. 



Arc between Oxides. As has been stated, the dis- 

 coveries of Auer and Nernst regarding the selective 

 emissive power of certain substances led to a further investi- 

 gation of the arc between other substances than carbon. 

 Among other substances were oxides similar to those in 

 the Nernst lamp. Rasch investigated these oxides and 

 gives the formula E = 31.35 + 30/7, the length of the arc 

 being i mm. 1 These oxides are non-conductors at ordi- 

 nary temperatures, and an auxiliary arc was used to heat 

 them until they became conductors. This arc has not 

 been developed into a commercial form. 2 



Measurements were made by Schultze 3 of arcs between 

 several oxides of metals, by placing the substance to be 

 examined in a cup hollowed out of a carbon electrode. He 

 found, in general, that the oxide having the lower melting 

 point requires the smaller E.M.F. when used as electrode. 

 With very short lengths of the arc he found the voltage to 

 be independent of the current. With greater length the 

 anode drop increased greatly with decreasing current and 

 the cathode drop increased little or none. 



Magnetite Arc. Ths most important of the arcs be- 

 tween oxides is the magnetite arc. 4 In this the cathode is 

 magnetite (Fe 3 O 4 ) and the anode is copper. Magnetite is 

 used for the cathode because it gives off a vapor which is 



1 Elektrot. ZS., 22, 55; 1901. 



2 See also Elektrot. ZS., 22, 256, 293, and 373; 1901. Verb. d. D. 

 Physik. Ges., 5, 157; 1903, and 6, 137; 1904. 



3 Ann. d. Phys., 12, 828; 1903. 



4 Elec. World, 43, 974; 1904. Trans. Intern. Cong., St. Louis, 2, 721; 

 1004. 



