INTRODUCTION 3 



that with the arc and the current as large, 1 and the only 

 distinction between these two forms of discharge is that in 

 the arc the cathode is heated by the current flowing through 

 the gas, while with the hot calcium oxide the cathode must 

 be heated by some external source. 



Though it is not possible to make a perfect distinction 

 between what is and what is not an arc, we shall not be far 

 from the common usage, if we define an arc as consisting 

 of a continuous current of several amperes or more, passing 

 through a gas and having a cathode drop which is com- 

 paratively small. The most common form is that between 

 carbons in air where the cathode drop is in the neighbor- 

 hood of 9 volts and the current 5 amperes or more. 



Appearance of Arc. There are probably few people in 

 civilized countries who have not seen an electric arc, and 

 yet it would be nearly as truthful to say that there are few 

 who have seen one. There are few who have not seen a 

 frosted globe which was intensely luminous and was said 

 to contain an electric arc, or who have not been blinded 

 by looking too closely at an uncovered arc. On the other 

 hand, there are few who have seen the difference between 

 the luminous gas and the even more luminous terminals, 

 who have seen the different shades of this gas as it varies 

 from the violet of the brilliant center to the yellow of the 

 faintly luminous edge, or have seen the fiery particles 

 sent off from the white hot carbons, but without seeing 

 these things one does not in reality see the electric arc. 



As far as it is possible to reproduce such a view by 

 means of an uncolored photograph, it is done in the 

 accompanying illustration, which shows an arc be- 

 tween carbon terminals. Unfortunately this does not give 



1 Phys. Rev., 29, 360; 1909. 



