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. SOURCES OF SOUND CURRENT 521
former coupling. Either may be used in connection with photo-electric cells, or between the tubes of amplifiers.
Figure 122 is a clear picture of a film reproducing system, which it is suggested that the reader study by way of review. The exciting lamp is shown without its light shield. The thumb-screws below and to the left of the lamp are the means by which the light beam is focussed to secure minimum height at the point of the sound-track. The photo-electric cell is not in the picture but is located in the compartment to the right of the sound gate. The film used for this photograph is blank leader (chosen because it will show up white), having neither pictures nor sound track.
Figure 123 shows a sound-head in which the beam of the exciting light is bent upward by the "Photo-cell condenser lens." The photo-cell, in other words, is not located in a straight line with the lens tube, as in Figure 117, but a bit above it. The light beam is bent upward after it has passed through the sound track.
Care of the Film Reproducing System
Everything about the film reproducing system is delicate, and requires extreme care. The focus of the exciting lamp is a matter of a fraction of a thousandth of an inch. The current of the photo-electric cell is a matter of a millionth of an ampere. And the slightest defect or irregularity in this part of the apparatus is subject to the full amplifying power of the sound system.
(20) The focus of the exciting lamp must be accurate. In the normal course of operation correct focus is not secured by adjusting the lenses or the slit, but by careful placing of the exciting lamp, the mounting of which can be moved until focus is obtained and then locked in place by set-screws. (21) A rough test for focus is to hold a white card in front of the photo-cell and adjust the focus until a clear white oval is obtained. A more accurate test is to run a loop of high-frequency test film, six or nine thousand cycles, while an output meter is con