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National Projector ( larhons
therefore, to use only the best quality of carbons, in correct combination and operated within the recommended range of current value.
As the motion picture industry developed, increases in the size of theatres, length of throw and dimensions of screen resulted in demands for more and more light from the arc. To meet this need for an enormous volume of projection light, the high intensity arc was developed, using National High Intensity White Flame Projector Carbons.
Low intensity D.C. arcs are operated at current densities of 120 to 165 amperes per square inch in the positive carbon. With SRA carbons, discussed in Chapter VII, current densities up to 205 amperes per square inch are reached. In the high intensity arc, however, current densities of 500 to more than 800 amperes per square inch are attained. This extreme current density requires a highly developed type of carbon of the greatest possible purity and the resulting arc is quite different from that of the low intensity types.
The light from the high intensity arc comes from two distinct sources; the crater of the positive carbon and the tail flame, Figure 21. The light from the tail flame represents about 30 per cent of the total light emitted from the arc but, since it cannot be focused within the dimensions of the aperture plate and any appreciable portion of it utilized by the optical system, the projectionist is interested only in the crater light. The whiteness and intrinsic brilliancy of this crater light exceeds that of incandescent carbon volatilized at atmospheric pressure. It evidently has its source in something more than the solid tip of incandescent carbon. This intensified light can be conceived as coming from a portion of the luminous gases of the tail flame, compressed in the crater of the positive carbon by the negative arc stream and thus augmenting the light from the incandescent carbon tip. This explanation may not accord with
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