PHOTOMETRY AND ELECTRIC LIGHTING. 343 



143. The carbon-filament glow lamp. The ordinary incandes- 

 cent lamp consists of a fine filament of carbon mounted in a 

 highly exhausted glass bulb through the walls of which platinum 

 lead-wires are sealed for connecting the carbon filament to the 

 supply mains. The flow of current through the filament heats it 

 to a high temperature and it emits a steady soft light that is well 

 adapted to inside illumination. 



The present process of manufacture is briefly as follows : Clean 

 cotton fiber is dissolved with constant stirring in an aqueous 

 solution of zinc chloride forming a thick syrup-like fluid which is 

 freed from lumps by filtering and from bubbles by heating nearly 

 to the boiling point under reduced pressure. This fluid is then 

 " squirted " through a small hole and the small gelatinous rod 

 is received in alcohol which quickly hardens it so that it can be 

 thoroughly washed in water, wound on a drum and dried. The 

 result is a fairly tough wire or thread of pure cellulose having 

 the appearance of celluloid. This thread is cut into lengths, 

 bent on formers into the desired shape of the lamp filament, 

 packed in charcoal, and carbonized in a furnace. The carbon- 

 ized filaments are then mounted on the platinum lead-wires and 

 heated electrically to a high temperature one at a time in the vapor 

 of a hydrocarbon such as gasolene. The hot filament decom- 

 poses the hydrocarbon vapor and causes a deposit of graphitic 

 carbon on the filament, thus increasing the size of the filament 

 slightly and lowering its resistance to the exact value desired. 

 This process is called "flashing." The finished filament is then 

 placed in the glass bulb which is exhausted and sealed.* 



The resistance of the carbon filament of a glow lamp is about 

 half as great when it is at its working temperature as it is at 

 20 C, the exact amount of variation depending upon the amount 



* See an article on incandescent-lamp manufacture, by Manning K. Eyre, The 

 Electrical World, January 5, 1895. 



A modified process of manufacture has recently been developed by the General 

 Electric Company. See a paper by J. W. Howell, " A New Carbon Filament," read 

 at the Ashville Convention of the Am. Inst. of Electrical Engineers, June, 1905, and 

 to be published in volume 24 of the Transactions. 



