NO. 2 



HISTORY OF ELECTRIC LIGHT SCHROEDER 



.47 



oxygen of the air, he coated it with oxide of zirconium. Unfortu- 

 nately zirconia, while an insulator at ordinary temperatures, becomes, 

 as is now known, a conductor of electricity when heated, so that the 

 lamp short circuited itself when it was lighted. 



Edison's High Resistance 

 Platinum Lamp, 1879. 



This lamp had a high resis- 

 tance burner, necessary for the 

 multiple system. 



Edison's High Resistance 



Platinum in Vacuum 



Lamp, 1879. 



This experimental lamp led 

 to the invention of the success- 

 ful carbon filament lamp. 



During his experiments he had found that platinum became ex- 

 ceedingly hard after it had been heated several times to incandescence 

 by current flowing through it. This apparently raised its melting 

 temperature so he was able to increase the operating temperature 

 and therefore greatly increase the candlepower of his lamps after 



