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MANAGERS AND PROJECTIONISTS 983
permit to pass through the film. Remember that the photo-electric cell will be active and set up electro-motive force in the amplifier circuit in exact proportion to the amount of light these shadings permit to reach it.
Considered in that light, there really is small mystery in the action of the sound track. It is made very clear, I think, in an article printed in Movietone Bulletin. I shall therefore reproduce it here, because I think it cannot be much improved upon. It is as follows :
"The pressure of sound waves impinging on the diaphragm of a condenser microphone (as distinguished from a carbon microphone), causes the diaphragm to vibrate. The amount of the displacement of the diaphragm is proportional to the intensity or loudness of the sound. This movement varies the capacity of the condenser microphone, thus altering the charge on the grid of a vacuum tube which is connected in series with the microphone. The resulting alternating current is proportional and in phase with the movement of the diaphragm.
"If a carbon microphone is used, the displacement of the diaphragm varies the resistance of the microphone and causes a varying or modulated direct current. This current may be considered as made up of a direct and alternating component.
"After several stages of amplification in which the voltage is increased approximately 100,000 times, it is impressed on an Aeo light. The Aeo light is a gas tube excited by direct current and so designed that its intensity or brightness is proportional to the applied voltage. The light shines through a narrow slit on a moving negative film in the camera and the variations of the light are recorded upon it. The exposure of the