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SOURCES OF SOUND CURRENT 523
until the maximum volume is shown by the registration of the meter.
The lenses of the slit assembly must be kept clean and free from oil. The outer lenses are cleaned with tissue paper, preferably lens tissue. In some makes of equipment the lens tube can be opened, cleaned and adjusted in the projection room, but other manufacturers seal their lens assemblies to prevent this practice. Some types of assembly can be focussed in the projection room, while in others the focus is fixed and the exciting lamp controls are the only means of modifying the focus.
(22) Oil should be kept away from the lens tube. It seeps inside, vaporizes under the heat of the exciting lamp, and turns the light yellow. The photo-electric cell does not respond well to yellow light. Loss of volume, especially at high frequencies, is experienced when the exciting light is dimmed in this way, and the only remedy is either to open and clean the lens assembly (if it can be opened in the projection room) or to return the assembly to the factory for overhauling.
(23) The sound gate, or tension pad, must be kept scrupulously clean. Particles of dirt or lint or film wax caught in the gate may vibrate with the motion of the film and add an unintended interruption to the light. This will result in noisy sound.
(24) The motion of the film past the sound gate must be utterly smooth. The slightest jerk will result in "flutter," which is a tremolo in the sound, and very unpleasant. The same result will follow if the sound gate does not hold the film tightly, but permits it to move forward and back through the point of focus. The motion of the film is controlled by the sprockets and idlers, which are described on another page under the heading of "Drives."
The "lateral guides" control the film on its downward course to insure that the entire sound track, and nothing but the sound track, passes through the exciting light. (25) There are sprocket holes in the film on one side of the sound track, and dividing lines between the pictures on the other side. If the film is laterally displaced so that