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366 O. M. GLUNT [j. s. M. p. E.
needle. From the standpoints of cost for records and of attention in operation the film record was selected in preference to the disk.
The question of replacements also arose in connection with the lamp and photo-electric cell which, as used in a sound picture projector, have an average useful life of 150 and 1500 hours, respectively. Due to the intermittent use of sound picture projector equipment, short-lived consumable items such as lamps are not a serious operative matter as long as they involve only a small cost. In the call announcer, due to their continuous use, the frequent replacement of these elements would present a serious operating obstacle quite apart from the cost of replacement. Fortunately, the needed signal level conditions of the call announcer made it possible to depart from the sound picture operating practice. This departure consists in operating the lamp at a much lower illumination level, obtaining a longer operating life for lamp and cell. As a result, the average life of these elements in a call announcer under normal conditions is about 200 days.
While the final form of the call announcer is much the same as in the first experimental model, a number of changes have been made. The photo-electric cell is now inside and the lamp and lens system are outside the drum, making it easier for the heat generated by the lamp to be carried away, thereby prolonging the life of the film. Other advantages of this arrangement are the greater accessibility of the various parts and the ability to use a greater amount of standard sound picture apparatus.
In its present form the apparatus includes a group of eight drums. These drums are arranged to carry four strips of sound film on each outer edge. One of these drums is a spare and each of the other seven carry two groups (4 strips each) of sound film ; a total of 14 groups. On each of the four strips of the 14 groups is recorded one of the ten digits or one of the four call letters J, M, R, and W. These drums are mounted on a common shaft and are driven by a small motor through a suitable reduction gear to give a constant peripheral drum speed of 90 feet per minute. In front of each film is a small lamp which is focused on the film by means of an optical system and behind each film and within the drum is a photo-electric cell.
The drum on which the film is mounted must be large enough to permit a photo-electric cell to be mounted within the shell. This requires a circumference much greater than that required for a single repetition of a number or letter; four announcements of the number or letter are therefore recorded, one on each piece of film, and these