14 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. "J^ 



platinum wire, the end below the tube being coiled. Another platinum 

 wire coiled at its upper end came up through the lower part of 

 the globe but did not quite touch the other platinum coil. The pow- 

 dered charcoal filled the two coils of platinum wire and bridged the 

 gap between. Current passing through this charcoal bridge heated 

 it to incandescence. The air in the globe having been removed as far 

 as was possible with the hand air pumps then available, the charcoal 

 did not immediately burn up, the small amount consumed being re- 

 placed by the supply in the tube. The idea was ingenious but the 



De Moleyns' Incandescent Lamp, i{ 



This consisted of two coils of platinum wire containing powdered 

 charcoal operating in a vacuum. It is only of interest as the first 

 incandescent lamp on which a patent (British) was granted. 



lamp was impractical as the globe rapidly blackened from the evapo- 

 ration of the incandescent charcoal. 



EARLY DEVELOPMENTS OF THE ARC LAMP 



It had been found that most of the hght of the arc came from 

 the tip of the positive electrode, and that the charcoal electrodes were 

 rapidly consumed, the positive electrode about twice as fast as the 

 negative. Mechanisms were designed to take care of this, together 

 with devices to start the arc by allowing the electrodes to touch each 

 other and then pulling them apart the proper distance. This distance 

 varied from one-eighth to three-quarters of an inch. 



