LIGHT, COLOR, AND SOUND 117 



leaves the lamp. The air is exhausted from the bulb as 

 completely as possible, and the exhaustion tube sealed off. 

 When the electricity passes through the filament, it glows 

 on account of the great resistance, but because of the lack 

 of air does not burn. The glowing particles of the filament 

 give off the illuminating rays. The way in which the light 

 is distributed from the lamp depends upon the form in which 

 the filament is bent. 



When certain metals with a very high melting point, such 

 as tungsten, osmium, etc., are made into fine wires or fila- 

 ments, they possess remarkable endurance and a high degree 

 of efficiency. 



118. The Nernst Lamp. The Nernst lamp has a filament 

 of compressed oxides of certain rare metals. This filament 

 conducts electricity only when heated to a high temperature, 

 and as it is not combustible it need not be enclosed in an 

 exhausted vessel. A small encircling coil of platinum wire 

 (called a heater) through which a current of electricity passes 

 brings the filament to incandescence. 



119. Arc Lamps. The ordinary arc light is formed be- 

 tween two carbons. When a current of electricity is passed 

 through these carbons, the great resistance offered causes 

 the ends of the carbon to become very hot and to glow. As 

 the carbon gradually burns, the distance between the ends 

 becomes greater. An automatic attachment by which the 

 lower carbon is raised, keeps the distance between them 

 constant. 



120. The Drummond Light. The Drummond light is pro- 

 duced by exposing small pieces of lime to ignition in a blow- 



