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CHAPTER XIII THE LIGHT SOURCE
The light source is a subject of very great importance, since it is the base upon which excellence in projection rests.
(I) At present there are only two sources of light available for projection, namely, the electric arc and Mazda. (2) There are several varieties of the electric arc, each of which is different from all other arcs used for projection and (3) each requires different treatment for production of the maximum quantity and quality of light.
(4) The electric arc is formed by charging two carbons with opposite polarities of E.M.F. from the same power source, bringing them into physical contact and again separating them. (5) Physical contact results in the flow of current which heats the carbon particles until they are "volatilized," that is, transformed into gases. (6) These gases form a high-resistance conductor between the positive and negative carbon tips over which current flows. (7) By the action of the heat engendered by the current, a saucer shaped depression is formed on the tip of the positive carbon (called the crater), the bottom or "floor" of which is, in (8) the straight carbon arc, heated approximately to 3,500 degrees Centigrade, about equal to 6,500 degrees Fahrenheit. (9) Except for the high intensity arc this is the highest brilliancy obtainable by heating any known solid substance. (10) Of all substances carbon will withstand the highest temperature before being transformed into gases. It is possible to secure higher brilliancy by combining chemicals with carbon, as will be explained later.
(II) The novice is often puzzled by the fact that carbon tips charged with E.M.F. may be advanced within a very small fraction of an inch of each other without
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