52 THE PRODUCTION AND USE OF LIGHT 



comes in contact with B f and A is in contact with A', com- 

 pleting the circuit. Connecting the two platinum wires 

 within the bulb is a filament sometimes composed of specially 

 prepared carbon. It is really a long thread of carbon. 

 When the current passes through, this filament is heated to a 

 white heat, or becomes INCANDESCENT (Fig. 37). There being 

 no air present, it cannot burn out. If air were present it 

 would be burned up in an instant. In most recent lamps 

 the filament is made of a rare metal called TUNGSTEN (Fig. 

 38). These lamps give much more light with far less current 

 than the lamps with the carbon filament. 



66. The Arc Lamp. The arc light is still more simple than 

 the incandescent. It consists of two pencils of carbon, each 

 about half an inch in diameter. The current flows in on one 

 of the pencils and out on ij^e other. Unlike the incandescent 

 filament these pencils are usually not protected from the air, 

 consequently they are consumed quite rapidly. At first the 

 carbons are permitted to rest against each other. A very 

 strong current of electricity is then sent through them. They 

 quickly become very hot at the points where they touch. 

 They are then separated about one-quarter of an inch. They 

 still continue to carry the current and glow, for the space be- 

 tween them 'is now filled with glowing-hot carbon vapor. This 

 glowing vapor is called the ELECTRIC ARC. 



Within this electric arc is the highest temperature ever 

 obtained by artificial means. It is the high temperature of 

 this arc which is used in the electric furnace to produce calcium 

 carbide from the lime and coke (Art. 58). It is estimated at 

 about 4000C. 



It is evident that, as the pencils are consumed, the space 

 between them will widen. This difficulty is overcome in most 

 lamps by an artificial feeding device which always keeps the 

 pencils at about the proper distance from each other. In 

 the ordinary open arc lamp the pencils are consumed at the 

 rate of about 1 inch per hour. 



The arc lamp is the most powerful of all artificial lighting 



