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394 RICHARDSON'S BLUEBOOK OF PROJECTION
the body of the carbon, thus avoiding heating the carbon by resistance. This has two other advantages. Since the positive is non-rotating, the contact clamp may be placed back a considerable distance from the crater. It may be a plain clamp, thus eliminating the machinery for operating a sliding contact and rotating the carbon, giving the mechanism great simplicity. Again, were all the current to pass through the body of the carbon, it would heat progressively toward the crater, developing a tendency to spindle-burn to a long, tapering point.
(152) As was noted before the crater of the high intensity arc light source must be cup-shaped and deep and of even height at all points so that the luminous gases generated therein by volatization will be retained. The gases are held in the crater by the impact of the negative gas stream.
This stream or flame must be at a slight angle to the face of the positive crater so that all surplus gas is compelled to leave the crater in an upward direction. When the negative is wrongly positioned the surplus gas escapes at various points on the rim of the crater, resulting in an unstable arc and poor screen illumination.
The proper condition is established in the regular high intensity arc by placing the negative carbon at an angle to the positive. (153) In the non-rotating carbon high intensity arc both carbons are centered on the axis of the optical system, hence lie approximately in horizontal position, but in order that the negative gas stream may strike the positive crater at the desired angle, the negative is located with its center a trifle lower than the center of the positive carbon. The action of the gas stream is then aided by forcing it upward by means of a magnet, because an electric arc is influenced by magnetic force.
Carbons in Line
(154) While the negative must be a trifle below the positive, both must otherwise be in exact alignment with each other. If they are not the crater lip will burn unevenly giving poor illumination.