Artificial Lighting 



175 



Catalogs illustrating the chief features of the mechanisms 

 may be obtained from the local light and power plant. 



In the arc lamp the carbons are arranged to meet where the 

 light is produced. When the electric current is turned on, the 

 carbons are drawn apart about one fourth of an inch. The 

 current meeting the resistance of the air heats the ends of the 

 carbons sufficiently to volatilize them. The carbon vapor 

 streams across the break between the ends, forming the arc. 

 The vapor is heated white hot by the electric current and 

 makes the arc luminous. 



The temperature at the crater of the arc at ordinary air pres- 

 sure may reach as much as 4000 to 4500 C. This is taken 

 as the melting or vaporizing point of carbon. It is interesting 

 to note that in recent experiments with the carbon arc under 

 different pressures unusually high temperatures have been 

 obtained. Under a pressure of about 20 atmospheres (what is 

 one atmosphere?) a brightness eighteen times that of the 

 ordinary arc has been obtained. This indicates a temperature 

 of about 7800 C. (What is the equivalent on the Fahrenheit 

 scale ?) It may be possible to make use of high pressure in arc 

 lamps. At such temperatures as a 



are obtainable with increased pres- 

 sure or even at that which is sup- 

 posed to be the sun's temperature, 

 about 6000 C., the light produced 

 would, within a limited area, equal 

 sunlight in quality and intensity. 



Inclosed arc. The carbons are 

 consumed very rapidly in the open- 

 air arc lamp. Recently the in- FIG. 5 1. The inclosed arc lamp. 



closed arc (Fig. 51) has proved to be more economical and 

 has driven the open-air arc practically out of the field. In 

 this type of lamp the arc is inclosed in a glass globe which 

 admits a very small amount of air and consequently the car- 

 bons last several times as long. 



