are consumed rather rapidly so they are made as long as possible.  In one type of arc, the carbons are both fed downward, their lower ends forming a narrow V with the arc-flame between their tips.  Under these conditions the arc tends to travel vertically and finally to “stretch” itself to extinction.  However, the arc is kept in place by means of a magnet above it which repels the arc and holds it at the ends of the carbons.

The chief objection to the early flame-arcs was the necessity for frequent renewal of the carbons.  This was overcome to a large extent in the Jandus regenerative lamp in which the arc operates in a glass enclosure surrounded by an opal globe.  However, in addition to the inner glass enclosure, two cooling chambers of metal are attached to it.  Air enters at the bottom and the fumes from the arc pass upward and into the cooling chambers, where the solid products are deposited.  The air on returning to the bottom is thus relieved of these solids and the inner glass enclosure remains fairly clean.  The lower carbon is impregnated with salts for producing the luminous flame and the upper carbon is cored.  The life of the electrodes is about seventy-five hours.

The next step was the introduction of the so-called “luminous-arc” which is a “flame-arc” with entirely different electrodes.  The lower (negative) electrode consists of an iron tube packed chiefly with magnetite (an iron oxide) and titanium oxide in the approximate proportions of three to one respectively.  The magnetite is a conductor of electricity which is easily vaporized.  The arc-flame is large and the titanium gives it a high brilliancy.  The positive electrode, usually the upper one, is a short, thick, solid cylinder of copper, which is consumed very slowly.  This lamp, known as the magnetite-arc, has a luminous efficiency of about 20 lumens per watt with a clear glass globe.

The mechanisms which strike the arc and feed the carbons are ingenious devices of many designs depending upon the kind of arc and upon the character of the electric circuit to which it is connected.  Late developments in electric incandescent filament lamps have usurped some of the fields in which the arc-lamp reigned supreme for years and its future does not appear as bright now as it did ten years ago.  High-intensity arcs have been devised with small carbons for special purposes and considered as a whole a great amount of ingenuity has been expended in the development of arc-lamps.  There will be a continued demand for arc-lamps, for scientific developments are opening new fields for them.  Their value in photo-engraving, in the moving-picture production studios, in moving-picture projection, and in certain aspects of stage-lighting is firmly established, and it appears that they will find application in certain chemical industries because the arc is a powerful source of radiant energy which is very active in its effects upon chemical reactions.