546 LIGHT FROM THE ARC [Cn. XIII 



THE LIGHT PRODUCTION OF THE ARC 



749. Cause of light from the arc. The light production from 

 the carbon arc is due entirely to the high temperature to which the 

 tips of the carbons are raised, i. e., they become white hot. The 

 practical problem in projection with the arc deals with the best 

 method of producing this white heat and of utilizing it. 



When the electric current passes between the two electrodes the 

 heating effect in the different parts is proportional to the power 

 consumed in them. 



The current being the same in all parts, the heating effect must 

 be in proportion to the potential drop (or voltage consumed) in the 

 different parts. 



Counting the total drop 55 volts, it is divided into: 



+ crater drop = 32 volts = 58% 



crater drop = 9 volts = 17% 



arc stream = 14 volts = 25% 



Total, 55 volts 100% 



We see from this that the heating effect will occur principally 

 at the positive carbon. 



Carbon being rather a poor conductor of heat, the heat generated 

 within the small area of the crater must escape mainly by radia- 

 tion. 



At the negative electrode the heat production is less rapid and 

 not so high a temperature is reached. 



Between the electrodes the heat production is fairly rapid, but 

 the hot gases of the arc stream with the carbon arc are nearly 

 transparent and radiate energy very slowly. 



Furthermore the violet lines of the spectrum in the arc stream 

 are brighter than from the crater itself ( 749 a). 



750. Temperature of the crater. The temperature of the 

 positive crater rises until such a temperature is reached that carbon 



749a. The great brilliancy of the violet lines in the arc stream has received 

 two explanations: (i) That the arc stream is higher in temperature than even 

 the crater itself; (2) That the electric current passing through the gas causes 

 the gas to glow irrespective of its temperature. That is, it causes electro- 

 luminescence as in the vacuum tube or the aurora borealis. 



