During the early years of incandescent lamps, improvements were made from time to time which increased the life and the luminous efficiency of the carbon filaments, but it was not until 1906 that any radical improvement was achieved.  In that year in this country a process was devised whereby the carbon filament was made more compact.  In fact, from its appearance it received the name “metallized filament.”  These carbon filaments are prepared in the same manner as the earlier ones but are finally “treated” by heating in an atmosphere of hydrocarbons such as coal-gas.  The filament is heated by electric current and the heat breaks down the hydrocarbons, with the result that carbon is deposited upon the filament.  This “treated” filament has a coating of hard carbon and its electrical resistance is greater than that of the untreated filament.

The luminous efficiency of a carbon filament is a function of its temperature and it increases very rapidly with increasing temperature.  For this reason it is a constant aim to reach high filament temperatures.  Of all the materials used in filaments up to the present time, carbon possesses the highest melting-point (perhaps as high as 7000 deg.F.), but the carbon filament as operated in practice has a lower efficiency than any other filament.  This is because the highest temperature at which it can be operated and still have a reasonable life is much lower than that of metallic filaments.  The incandescent carbon in the evacuated bulb sublimes or volatilizes and deposits upon the bulb.  This decreases the size of the filament eventually to the breaking-point and the blackening of the bulb decreases the output of light.  The treated filament was found to be a harder form of carbon that did not volatilize as rapidly as the untreated filament.  It immediately became possible to operate it at a higher temperature with a resulting increase of luminous efficiency.  This “graphitized” carbon filament lamp became known as the gem lamp in this country and many persons have wondered over the word “gem.”  The first two letters stand for “General Electric” and the last for “metallized.”  This lamp was welcomed with enthusiasm in its day, but the day for carbon filaments has passed.  The advent of incandescent lamps of higher efficiency has made it uneconomical to use carbon lamps for general lighting purposes.  Although the treated carbon filament was a great improvement, its reign was cut short by the appearance of metal filaments.

In 1803 a new element was discovered and named tantalum.  It is a dark, lustrous, hard metal.  Pure tantalum is harder than steel; it may be drawn into fine wire; and its melting-point is very high (about 5100 deg.F.).  It is seen to possess properties desirable for filaments, but for some reason it did not attract attention for a long time.  A century elapsed after its discovery before von Bolton produced the first tantalum filament lamp.  Owing to the low electrical resistance of