330 



ELEMENTS OF ELECTRICAL ENGINEERING. 



137. Volt-ampere characteristics of the electric arc.* --The 

 electric arc, whether it be between rods of pure carbon, or rods 

 of carbon impregnated with metallic salts, or rods of meta^ 

 always has this property that the greater the current the less the 

 voltage across the arc, except when the arc is very short. Thus 

 the coordinates of the curves in Figs. 182 and 183 give some 

 idea of the relation between the current that flows in an arc and 

 the voltage across the arc. The exact relation between current 

 and voltage depends upon the diameter of the carbon rods, upon 

 the degree of purity of the carbon, upon the texture of the car- 



2 5 Amperes 



Fig. 182. Volt-ampere characteristics of an open direct-current arc between cored carbons one 

 half inch in diameter. 



bon, upon the freedom of circulation of the surrounding air, and 

 upon the nature of the current (direct or alternating), but the re- 

 lations shown in Figs. 182 and 183 are typical. The numbers 

 attached to the various curves indicate the length of the arc in 

 inches. 



When the current flowing in an arc is increased, the arc, that 

 is to say the conducting column of heated vapor, increases in sec- 



* For a discussion of the physics of the electric arc, see C. P. Steinmetz, Trans. 

 International Electrical Congress, Vol. II., pp. 710-730, St. Louis, 1904. Also see 

 W. R. Whitney, Trans. American Electrochemical Society, Vol. VII., pp. 291-299, 

 1905. 



