ILL!'. \H\.\Tln\ AM) rifnro.M KTIfY 421 



at these high temperatures. Also, the electrical resistivity of the 

 filament material should he so high that the length of the filament 

 for a given resistance shall not be excessive. For man}' years 

 the carbon filament was the only one that proved satisfactory. 

 It con-ists of a filament of carbonized cellulose, bent into horse- 

 shoe form and operated in a vacuum. A later development was 

 the "G.E.M." lamp in which the carbon is "metallized/' that 

 is, it is flashed in a gas rich in hydrocarbons. This gives the 

 carbon filament a metallic appearance and a positive tempera- 

 tun coefficient such as metals have, and permits the filament to 

 operate at a higher temperature. 



The purpose of a vacuum in a lamp is two-fold. A vacuum 

 prevents chemical action on the filament and is an excellent heat 

 insulator. 



The treated carbon-filament lamp has an efficiency of about 

 3.0 watts per mean horizontal candlepower and the metallized 

 filament an efficiency of about 2.5 watts per mean horizontal 

 candlepower. 



The objection to the carbon-filament lamp is its low efficiency. 

 Remembering that theamountof lightemitted by an incand< 

 source increases as the fourth power of its absolute 4 temperature, 

 carbon-filament lamps can be made to have very high efficiency by 

 operating them at high temperatures. However, this increased 

 efficiency is accompanied by rapid evaporation of the filament, 

 resulting in a very short life. Carbon-filament lamps are there- 

 fore operated at such a temperature that their life is from 700 to 

 1,000 hours. Their candlepower may become so reduced after 

 'hat it is more economical to discard the lamps than 

 to continue their use at this low candlepower. 



188. The Tantalum Lamp.-- The metal tantalum was next 

 used as a lamp filament. Because of the high temperature at 

 which tantalum can safely operate, tantalum lamps are able to 

 develop an efficiency of 2.0 watts per mean hori/ontal candle- 

 power and at the same time have an average life of about 600 

 hours. One peculiarity of the lamp is that when alternating 

 curre: -I the filament disintegrates rapidly. < >\\ini: to the 



1 by the tungsten lamp. I he tantalum lain)) 

 ha- been absolutely superseded. 



189. The Tungsten Type-A and Type-B Mazda Lam 

 -ten is a metal of high re>i>tivity and is capable of with- 



