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Moisture in Motion Picture Film — Sease
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rapidly enough they continue to build up until a discharge takes place and brings about a neutralization. The discharge may be through the air in the small angle where the film is leaving the roll, or through the perforations, or over the edge of the film, giving rise to the familiar static markings after development. The above experiments also indicate that a static charge would be generated not only at the point where the film is leaving the roll but at every point where it is breaking contact with rollers or gates.
Percent Relative Humidity^
Fig. 5. Curve showing variation with humidity of static electrical charges produced on film base.
In Fig. 5 is shown the variation in the magnitude of the charges produced on cellulose nitrate film base in equilibrium with atmospheres at different humidities when it was moved out of contact with tin-foil. The charge is expressed in arbitrary units. There was a rapid drop in the magnitude of the charge after passing above 60% relative humidity. The electrical conductivity of the film base used in this experiment was measured. There was a rapid increase in the volume conductivity of the base with increase in moisture content. The results seemed sufficient to explain the rapid drop in the magnitude of the charge as shown in Fig. 5 and to justify the conclusion that the charges, which tend to collect on the opposite sides of a roll of film when it is unwound or passed over rollers, are neutralized at the proper moisture content by leakage directly through the body of the film.