INTRODUCTION 5 



There is a point on the cathode which is also intensely 

 luminous, but this is much smaller than the crater, so that 

 much less light is received from it than from the anode. 



The gas between the carbons is luminous and seen by 

 itself would be considered brilliantly so, but compared 

 with the crater it gives but little light. This at least is 

 true of the form of the arc shown in the accompanying 

 illustration. In the flaming arc, which is now coming into 

 use, the vapor gives out more light than the crater. In 

 that form the distance between the electrodes is longer 

 and the vapor is more luminous than in the older form, 

 and the crater less so. 



The light from the vapor is not due to incandescent 

 particles, but to the luminosity of the gas itself. This is 

 shown by the fact that its spectrum is a line spectrum. 

 The different parts of the vapor have different colors, 

 which will be considered more in detail in connection with 

 the spectrum of the arc. 



Discovery of the Arc. As has been stated, it is im- 

 possible to give the date of the first arc, because no at- 

 tempt was made to distinguish between the arc and the 

 spark. No mention was made by the first experimenters 

 of the length of time occupied by the wonderful flash they 

 described, and no one measured the current. The con- 

 fusion between the two forms of discharge was the more ex- 

 cusable on their part, since they were trying to show that 

 the electricity from chemical action was the same as that 

 produced by friction and gave the same kind of flash, but 

 it makes it inconvenient for one who tries to write a history 

 of this work. 1 



1 The following is a list of a few of the articles appearing in the early 

 part of last century: 



