142 THE ELECTRIC ARC 



the others, and the value of a lamp for street lighting de- 

 pends very largely on the amount sent in this direction. 

 This is partly responsible for the fact that the open arc has 

 been superseded to a very great extent by other forms. 



The figures for the flaming arc show a very low value of 

 the watts per candle power and also a very desirable dis- 

 tribution of the light. In the first of these respects it excels 

 any other form of light. 



In 1908 a report was submitted to the German Govern- 

 ment 1 in which they gave the light intensity in hefner units 

 per kilowatt for different sources of light as follows: 



Common form incandescent filament lamps 300 



Improved incandescent filament lamps 500 



Nernst and tantulum incandescent lamps 500 



Osram and tungsten from 700-1000 



Alternating-current arc between cored carbons 800 



Direct-current arc between cored carbons 1000 



Direct-current flaming arc 2000 



Mercury vapor arc 1600 



Mercury vapor arc in quartz tube 3000 



Spectrum of the Arc. The spectrum of the terminals 

 of the arc is, of course, the continuous spectrum of an in- 

 candescent solid and does not differ from that of other 

 incandescent solids. 



Unfortunately, no such simple statement can be made 

 concerning the spectrum of the gaseous part. Instead of 

 some simple spectrum of carbon monoxide, for example, 

 with an occasional line due to some impurity in the carbon, 

 there is one so complicated that it is not possible to deter- 

 mine the cause of all the lines. There are not only lines due 

 to all the impurities which would be expected, but many 

 due to impurities which would not be expected. 



1 La Revue Elec., Jan. 15, 1909. 



