54 THE ELECTRIC ARC 



electrodes, of which the magnetite arc is the best known; 

 and arcs between metals, of which the mercury arc in a 

 vacuum is the only commercial example. 



All of these arcs require a lower voltage for the same 

 current and length than the carbon arc, and they all give 

 more light for the same amount of energy. It is also true 

 that each has some disadvantage. Many compounds 

 which might be introduced into the arc give fumes which 

 make them disagreeable in an enclosed room, and some 

 blacken any globe which may surround the arc. Many 

 give colors which are better suited for advertising purposes 

 than for common use. Some combinations of salts give 

 trouble either with regulating the lamp, or with starting 

 the arc after it has been extinguished. 



Unfortunately investigation of these arcs must be en- 

 tirely empirical, for we know nothing of the laws of radi- 

 ation from mineral vapors at high temperatures, nor of the 

 laws governing the relation between electrical excitation 

 of an atom and its emission of light. We can not reason 

 out in advance the amount of light to be expected from a 

 given substance. An analysis of the spectrum of differ- 

 ent vapors does not always help us. The spectrum of 

 calcium fluoride, for example, has no resemblance to that 

 of other salts of calcium and is more brilliant. 



Flaming Arc. The general appearance of the first of 

 these types, namely, those whose electrodes are carbon 

 with certain salts mixed with them, is quite different from 

 that of the arc between pure carbons. This is indicated by 

 Fig. 14, which is reproduced from a photograph of a "flam- 

 ing arc," as this class of arcs is called. This arc is very 

 much longer than the older form, and the greater part of 

 the light comes from the vapor instead of from the carbon. 



