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Crabtree and Ives — Static Markings on Film 69
an effect on the intensity of the static markings produced by a given discharge as might be expected.
Factors Affecting the Quantity of Static Electricity Produced on Motion Picture Film
In motion picture work, electrical excitation of motion picture film is largely produced by rubbing. The quantity of electricity produced depends upon the following factors : 1. The Electrical Conductivity of the Substances Rubbed
A . The Conductivity of the Film Base
If a good conductor of electricity such as a metal is insulated and subjected to friction, an electrical charge is generated which distributes itself more or less evenly over the surface, depending on its shape, and if the metal is grounded by connecting to the earth, the whole of the charge flows away. In view of this tendency of the electricity to distribute itself over the conductor, it is difficult to generate a charge of sufficiently high potential to produce a disruptive spark on discharging. In the case of a nonconductor the charge remains where it was generated and if grounded at any one spot it is discharged only locally.
Therefore, if the conductivity of a substance is increased it has less tendency to develop a high potential locally, that is, there is a close parallelism between the electrical conductivity of a substance and the propensity for it to give static discharges. This relation is seen in the comparative tendency of a dried film of gelatine emulsion, motion picture negative film base, and ordinar}^ nitrocellulose base, to generate static electricity. The surface electrical conductivity of the materials is roughly in the order given and the tendency to produce static in the inverse order.
Although a strip of comparatively dry gelatine emulsion will generate static, the quantity produced is so slight as compared with that produced under the same conditions on the film base as to be of negligible importance in practice, so that it is usually only necessary to consider the film base.
By special treatment of the film base its conductivity may be increased to such an extent that its tendency to generate static is very much less than the untreated base.
Since gelatine and a gelatine emulsion are much better conductors than film base it would be expected that double coated motion picture positive film such as is used in subtractive color photography, and gelatine backed film such as non-curling roll film would have a much less tendency to generate static than untreated nitrocellulose film base, and this has been found to be the case. The film conductivity can also be increased and its tendency to generate static thereby decreased by increasing the moisture content as described later.