es during the recent war.

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