48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



they had been heated a few times. Examination of the platinum 

 under a microscope showed it to be much less porous after heating, 

 SO he reasoned that gases were occluded throughout the platinum 

 and were driven out by the heat. This led him to make a lamp with 

 a platinum wire to operate in vacuum, as he thought that more of the 

 occluded gases would come out under such circumstances. 



These lamps were expensive to make, and, knowing that he could 

 get the requisite high resistance at much less cost from a long and 



Edison's Carbon Lamp of October 21, 1879. 



This experimental lamp, having a high resistance carbon filament 

 operating in a high vacuum maintained by an all-glass globe, was 

 the keystone of Edison's successful incandescent lighting system. All 

 incandescent lamps made today embody the basic features of this 

 lamp. This replica is in the Smithsonian Institution exhibit of Edison 

 lamps. The original was destroyed. 



slender piece of carbon, he thought he might be able to make the carbon 

 last in the high vacuum he had been able to obtain from the newly 

 invented Geissler and Sprengel mercury air pumps. After several 

 trials he finally was able to carbonize a piece of ordinary sewing thread. 

 This he mounted in a one-piece all glass globe, all joints fused by melt- 

 ing the glass together, which he considered was essential in order to 

 maintain the high vacuum. Platinum wires were fused in the glass to 

 connect the carbonized thread inside the bulb with the circuit outside 

 as platinum has the same coefficient of expansion as glass and hence 

 maintains an airtight joint. He reasoned that there would be oc- 

 cluded gases in the carbonized thread which would immediately burn 



