CH. XIII] LIGHT FROM THE ARC 547 



is volatilized. This is the highest temperature which it is possible 

 to obtain artificially. The temperature of the positive crater of the 

 carbon arc has been estimated at about 3700 absolute, that is, 

 3427 Centigrade or 6200 Fahrenheit ( 7Soa). Compare this 

 with the temperature of the sun, about 6750 absolute, 6477 C; 

 the acetylene flame, 2330 absolute, 2057 C.; the gas flame, 1830 

 absolute, 1557 C. ( 75ob). 



751. Parts of the light source. Considered as a light source, 

 the direct current arc may be divided into four parts. 



1. The positive crater. 



2 . The negative crater. 



3. The hot ends of the carbons adjacent to the craters. 



4. The arc stream. 



The light emitted by the hot electrodes depends upon their vis- 

 ible radiation being approximately proportional to the 5th power 

 of their absolute temperature. The positive crater is the hottest 

 part of the arc and furnishes most of the light. The negative crater 

 furnishes much less light than the positive crater, being smaller and 

 not as hot. 



The carbons are white hot for some distance away from the 

 craters and furnish some of the light of the arc. In calculating the 

 total light from the arc it would be necessary to consider the entire 

 area included between the line surrounding the positive carbon 

 which is at red heat and the corresponding line on the negative 

 carbon. 



The arc stream with the carbon arc emits but little useful light. 

 When flame-arc carbons are used, however, the greater part of the 



750a. Bulletin of the Bureau of Standards, Vol. i, p. 909 and reprint 8. 



750b. Absolute temperature. The absolute zero is defined as the tem- 

 perature at which a perfect gas would exert no pressure. This is about -2 73 

 centigrade, i. e., 273 centigrade below the melting point of ice. In calcula- 

 tions of high temperature and radiation, all formulae are based on absolute 

 temperature, that is, the temperatures where the zero is the absolute zero and 

 where the degree is the degree centigrade. 



To find the absolute temperature of a body add 273 to its temperature on 

 the centigrade scale. Thus ice melts at o centigrade or 273 absolute, and 

 water boils at 100 centigrade or 373 absolute. The temperature of the 

 human body, 37.5 C. is 310.5 absolute. If the absolute temperature is given, 

 subtract 273 from this value to find the centigrade reading. 



