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Crahtree and Ives — Static Markings on Film 73
Although tests were not made with sheets of emulsion stripped from the base, comparative tests made by rubbing gelatine sheets and the emulsion side of motion picture film exposed to the same atmosphere, showed that positive and negative motion picture emulsions have less tendency to generate static electricity than plain gelatine.
The above tests also show that with motion picture negative film the air must have a relative humidity of about 90% at 50° F. and about 85% at 110° F. if it is to entirely prevent the generation of static electricity when the film is exposed to it for a few hours.
Since with air at any constant relative humidity the quantity of water which it contains increases with rise of temperature, film in equilibrium with such air contains a greater quantity of water at higher temperatures. Since the propensity of film to give static markings runs parallel with the absolute quantity of moisture which it contains, it would be expected that at a given relative humidity the propensity of film to give static would decrease with rise of temperature, as was shown by the above experiments.
(b) A dry emulsion or a dry film base absorbs moisture comparatively slowly. Bone dry motion picture film must be humidified for more than 24 hours in an atmosphere at 80% to 90% relative humidity before it absorbs all the moisture it will hold under these conditions. Hence, the condition of the air has very little effect unless the film is exposed to it for a sufficient length of time. Thus, dry motion picture positive film may give static markings even if the air of the printing room is saturated, if the film is not given an opportunity to absorb moisture. On the other hand, film containing an excess of moisture will not give static markings when immediately placed in dry air.
The fact that motion picture film is usually tightly rolled also hinders the rapid attainment of equilibrium with the atmosphere, but this is advantageous, in case film has to be stored in a dry atmosphere. If conditions are such that static markings are produced on positive film in the laboratory, in order to further humidity the film in the roll it must be stored for several weeks in a moist atmosphere, but not one which is too moist, otherwise the edges of the film will stick together and on unwinding more static will be produced than if the film was handled in its original condition.
Nature and Classification of Static Markings
Static markings produced directly on an emulsion are invariably black, and in the case of a negative, they print as white markings on the positive print. The markings frequently occur at regular intervals owing to the intermittent movement of the film in the camera or printer gate (see Fig. 10), although more often the occurrence is at irregular intervals.
If the friction on the film is local the discharge usually takes place in the same vicinity, but if the friction is evenly distributed over