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1949 DRUM-TYPE CAMERA 493
each mounted on the shafts of the identical electric motors (28) and (29). In a similar fashion gears attached to the lower ends of the motor shafts drive the film drum and the prism assembly; the sectioned film drum can be clearly seen in the bottom chamber, but the prism assembly, somewhat behind the plane of sectioning, is partially obscured by the details of the drawing.
The slotted disk and the film drum are made from forged blanks of high-strength Duraluminum. The gears are made from nickel alloy and tool steel; their teeth are cut on 15-degree helices and have a normal pitch of 32 teeth per inch of diameter. Each elementary prism in the 10-prism "stack" is of stainless steel and has 20 faces polished to a high degree of optical flatness. The electric motors are rated to deliver 16 horsepower each at 18,000 revolutions per minute; they are of the 3-phase, 2-pole type and are driven by a variable-speed alternator. The speed of the machine is measured from the frequency of the electromotive force induced in the coil (50) by the revolving permanent magnet (49) attached to the lower end of the film-drum shaft.
The picture rate for which the camera was designed, namely 200,000 frames per second, requires that the prism assembly rotate at 60,000 revolutions per minute, the slotted disk at 30,000 revolutions per minute, and the film drum at 9000 revolutions per minute.
METHOD OF TAKING PICTURES
Since the picture-taking rate is high enough to expose the whole length of the film in Vioo second or less, ordinary magnesium wire flashbulbs, whose effective flash duration is of the same order, can be used to illuminate the subject. These bulbs are mounted in parabolic reflectors in numbers depending on the picture rate and the reflectance of the subject. To take a sequence of pictures the camera is accelerated up to the required speed and when that speed is reached a timing device trips the camera shutter, the flashbulbs, and the event at the proper instants.
PICTURES TAKEN BY THE CAMERA
Fig. 4 shows an enlarged portion of a length of film as it comes from the camera. It contains a sequence of pictures of a revolving disk on which a series of parallel lines and an arrow indicating the direction of rotation have been drawn. From the increments of rotation between successive frames it can be seen that time is increasing from left