CHAPTER III. 



ARC IN AIR BETWEEN OTHER SUBSTANCES THAN CARBON. 



So far we have considered only arcs between terminals 

 of nearly pure carbon. As has been stated it is possible to 

 use for terminals many other substances. Attempts were 

 made in the early part of the last century either to use 

 a metal or to add some substance to the carbon which 

 would increase the efficiency of the arc for giving light. 

 These attempts did not at the time meet with success. 

 They led rather to the belief that the substance which 

 could be raised to the highest temperature without being 

 melted was the most efficient, and since carbon can be 

 raised to a higher temperature than any other material 

 which is commercially feasible, it was believed to be the 

 best for electrodes. 



But the discoveries of Auer and Nernst showed that 

 there are materials which have an emissive power more or 

 less selective and that an ideal black body is not neces- 

 sarily the best for emitting light. It, therefore, became 

 evident that there might be some better substance which 

 could be used for the electrodes of the arc. Several of 

 those which have since been tried have met with a large 

 degree of success. There are now three types of arcs 

 which embody some improvement over the carbon arc. 

 These are arcs having electrodes which are principally car- 

 bon, but have also a mixture of other substances, such as 

 .the "flaming" arcs; arcs having an oxide for one of the 



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