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Carbon
tivity, infusible and slow burning at the extreme temperature of the arc, carbon has proved to be the ideal electrode material for this purpose.
The carbon arc lamp is used for general illumination to a more limited extent than was formerly the case but the development of new types of carbons to meet special applications has extended its use into other fields where it has shown marked superiority over other sources of illumination. In the home — as well as in hospitals and sanatoria, the carbon arc is used to produce artificial sunlight and radiation of specialized character which physicians practicing light therapy have found valuable in the prevention and cure of certain physical disorders. The carbon arc has become an important tool of industry for processing materials by means of photo-chemical reactions and for accelerated tests of materials which tend to deteriorate under the action of sunlight. The most powerful searchlights and aviation field flood lights are highly developed types of carbon arc lamps using carbons designed for this specific purpose. Thousands of carbon arcs are in daily use in photography, photo-engraving, blue printing, and allied industrial processes. The motion picture industry would never have reached and could not maintain its present high plane without the aid of the carbon arc which is used both for studio illumination and for the projection of motion pictures on the screen. The large screen, long throw, and high level of screen illumination in the modern theatre require an intensity in the light source that only the carbon arc can supply — a crater brilliancy sixteen million times the brilliancy of the screen.
The demands made upon projector carbons are extremely severe and the present high quality of National Projector Carbons has only been attained by painstaking research, backed by years of manufacturing experience. Great care is necessary in selecting raw materials of the highest possible purity