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78 Transactions of S.M.P.E., August 1925
1. By avoiding friction. All sprockets should be of correct dimensions and in alignment with the take-up roll. If the sprocket teeth are staggered, or if the take-up roll is in malalignment, excessive tension is exerted on one edge of the film. Too much tension should also be avoided at the take-up roll, while the loops should be adjusted to prevent any possibility of the film rubbing against itself or any part of the machine.
The printer should also be correctly "timed," that is, the pressure plate should be released before the pull-down movement commences and should not return in place before the film comes to rest. Although glass is not an ideal material for pressure plate construction in view of its nonconductivity, metal plates are unsatisfactory where a transparent plate is otherwise desired, while glass produces a minimum of scratches on the film. The pressure plate should be renewed whenever the surface becomes roughened.
2. By humidifying the film. When motion picture positive film leaves the factory it is in equilibrium with an atmosphere of 70 to 75% relative humidity, but if the laboratory conditions are favorable for the production of static markings the quantity of moisture which the raw film contains is not sufficient to positively insure the absence of static during processing. It would be dangerous, however, to humidify the film further during manufacture, owing to the danger of the formation of moisture spots when the film is stored.^ Since a certain lapse of time is necessary for moisture to affect the emulsion, it is possible to humidify film immediately previous to or during processing to an extent which would be dangerous if the film was to be subsequently stored.
3. By humidifying the air in the printing room. If the printers were always in perfect adjustment and not run at too high a speed, a higher relative humidity than 75% at 70° to 75° F. would not be necessary in the printing room. In order to take care of the excessive friction to which the film is liable to be subjected if the printers get out of adjustment it is advisable to maintain the relative humidity at from 80% to 90% at 70° to 75° F. At such a high relative humidity the air feels uncomfortably cool to the worker at temperatures below 68° F. and oppressively warm above 75° F.
The exact relative humidity to be maintained depends on the particular machines used, the condition of the film, the temperature of the air, and time during which the film is exposed to the air before it is subjected to friction. The higher the temperature the lower is the relative humidity necessary to overcome a given tendency for static.
Usually the film is exposed to the air for only a few seconds before reaching the printer gate. This period may be prolonged by looping the film over several idler rollers before it reaches the gate. Such a procedure, however, is usually unnecessary if the negative is humidified us described below.