Artificial Lighting 165 



Oil and ointment, and wax and wine, 



And the lovely colors called aniline ; 



You can make anything from a salve to a star, 



If you only know how, from black coal-tar." 



II. ELECTRIC LIGHTING 



63. Types of electric lamps. Lighting by electricity is 

 rapidly increasing in all parts of our country. This kind of 

 lighting offers many interesting points for study, some of 

 which may be taken up here for introductory consideration. 

 There are three well-known and quite distinctive types, the 

 incandescent, the arc, and the vacuum tube. We shall begin 

 with the study of the incandescent lamp. 



Exercise. Examine a lamp closely ; if a worn-out bulb is avail- 

 able, it may be examined. To open it without danger to oneself and 

 to the filament, heat it near the base with the flame of a blowpipe. 

 When the glass becomes soft enough, it will be forced in gradually 

 by the air. Collect the parts of the filament for examination. Heat 

 a piece in a hot flame. Does it burn? 



64. The filaments. Filaments were formerly made of 

 bamboo or cotton- wool ; but to-day they are mostly of metals, 

 as tungsten, tantalum, thorium, and cerium. The bamboo or 

 cotton-wool filament was converted into carbon and then 

 placed in the glass bulb. Tungsten is rapidly displacing the 

 carbon filament. The original tungsten filaments were very 

 fragile, but with recent improvements in the preparation of 

 the metal for this purpose the filaments are now tough and 

 durable and the tungsten lamps may be handled as freely as 

 the carbon incandescent lamp without undue risk. 



Observe the way in which the filament is supported and 

 arranged in such a lamp as the "Mazda" (Fig. 47). Several 

 lateral wires are fused in each end of the glass rod which is a 

 continuation of the glass, through which the conducting wires 

 pass. A single continuous filament of tungsten is wound up and 

 down on these supports. The ends of the filament are fused to 

 the respective ends of the conducting wires. 



