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In the early days of the motion picture industry, the source of light was, in many instances, of the well known calcium type. Artificial gas, such as acetylene was also used, but as the public became interested in this new form of entertainment and education, the demand for a better form of illuminant necessitated discarding these sources of light. This, was due to the increased size of the picture theatres and the demand for larger and better illuminated or more distinct pictures. It was for these reasons, primarily, that the electric arc was adopted for motion picture work.
The electric arc is particularly well adapted to this service as it is one of the most intensely brilliant artificial illuminants known to science. The major portion of the light produced, however, is not obtained directly from the arc, but from the incandescent tips of the carbon electrodes. The brilliant illumination of the film is secured by the condenser lenses which collect a portion of the light produced 2nd converge it through the aperture plate, the illuminated area being generally known as the "spot." It is obvious, therefore, that if the spot is to be uniformly bright over its entire area the source of light must be concentrated in a fixed and as small a point as possible.
1. (a) The earliest current supply used for an electric arc for a light source for projection work was alternating, due to the fact that alternating current is more widely distributed and therefore, more readily available, and also because control apparatus for alternating current was simpler, m^ore easily designed and less expensive. It is practically impossible to find direct current mains everywhere, except in the centers of some of the large cities, while alternating current is available at practically all places where there is a sufficient number of people to warrant the establishment of motion picture houses. The apparatus for the control of alternating current for an alternating current arc is simple, comparatively inexpensive, and on the market in a variety of forms readily available for practically any current throughout the range of application. The advantage of alternating current from a carbon standpoint lies only in the efficiency of the supply to the arc. By means of a transformer, reactance, etc., the line voltage can be very readily reduced to any value required by the arc with very little loss of power.
1. (b) That the arc produced by alternating current is not so satisfactory as the arc produced by direct current is quite generally recognized. This fact is due to a number of physical characteristics of the alternating current arc. (1) Both electrodes are heated almost uniformly, producing practically equal illumination. However, only the incandescent tip of one carbon can be located at the proper focal point for the condenser lenses. (2) The average temperature of the crater is comparatively low on account of the rapid transfer of this crater from one carbon end to the other. (3) If an improper revolving shutter is employed or if a satisfactory shutter is operated at other than correct speed, the pictures will have an undesirable flicker, due to the fact that the value of the alternating current is zero twice