THE BANISHMENT OF NIGHT 



could not be bent into the tortuous shapes possible with 

 platinum. Later various modifications in the shape 

 of the filament were made, usually as adaptations to 

 changes in the shape of the bulbs. 



At the same time that Edison was succeeding with 

 his bamboo carbon filaments, J. W. Swan had been al- 

 most as successful with a filament formed by treating 

 cotton thread with sulphuric acid, thus producing a 

 "parchmentized thread," which was afterwards car- 

 bonized. A modification of this process eventually 

 supplanted the Edison bamboo filament; and the fila- 

 ment now in common use the successor of the " parch- 

 mentized thread" is made of a form of soluble cellulose 

 prepared by dissolving purified cotton wool in a solution 

 of zinc chloride, and then pressing the material out into 

 long threads by pressing it through a die. 



The long thread so obtained is a semi-transparent 

 substance, resembling catgut, which when carbonized 

 at a high temperature forms a very elastic form of carbon 

 filament. To prepare the filament the cellulose threads 

 are cut into the proper lengths, bent into horseshoe 

 shape, double loops, or any desired form, and then folded 

 round carbon formers and immersed in plumbago 

 crucibles. On heating these crucibles to a high tempera- 

 ture the organic matter of the filaments is destroyed, 

 the carbon filaments remaining. These filaments are 

 then ready for attachment to the platinum leading-in 

 wires, which is accomplished either by means of a car- 

 bon cement or by a carbon-depositing process. They 

 are then placed in the glass bulbs and the wires her- 

 metically sealed, after which the bulbs are exhausted, 



[233] 



