Richardson's handbook of projection (1930)

984 HANDBOOK OF PROJECTION FOR film is in phase and proportional to the alternating voltage applied to the Aeo light. Thus the sound track is a record of the exposure due to the variations in brightness of the Aeo light. "In reproducing, the printed sound track is passed through a narrow light beam, (.001" x .08"), of constant intensity and the transmitted light falls on a photoelectric cell. With proper developing and printing the light transmitted through the print is proportional to the exposure of the negative at the corresponding point. The resistance of a photo-electric cell varies inversely proportionately to the light shining on it. Thus the current flowing through the cell is proportional to the light transmitted through the film. The voltage drop across a series resistance in the cell circuit is impressed on the grid of the first tube of an amplifier. After it has been amplified approximately 100,000 times, this voltage is impressed on a loud speaker and the diaphragm moves proportionately. As the amplification is accomplished without appreciable distortion, the movement of the diaphragm of the loud speaker is proportional to the movement of the diaphragm of the microphone and the sound which was recorded is reproduced. "We are now in a position to analyze the sound track. If the source of the sound to be recorded has a frequency of 60 cycles, the Aeo light intensity will rise to a maximum, fall to a minimum and return to normal 60 times every second. The film moves 90 feet per minute, or 18 inches per second. Therefore, on every 18 inches of film there will be found 60 light and 60 dark striations, .15 inches wide. If the frequency were increased to 6000 cycles the width of the striations would