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Crahtree and Ives — Static Markings on Film 75
in a step printer. The intermittent occurrence of these is shown in Fig. 11.
In Fig. 6 the lower half of the fan-shaped marking is of much less density than the upper half and not so sharply defined, and is probably a result of either a reflection of the upper discharge, or a secondary weak discharge.
S. Miscellaneous markings. Those shown in Fig. 9 were produced on bone dry negative film in a camera and consist of a conglomeration of dots, branches, and fans.
Static Markings Encomitered in Practice and Methods of Their
Prevention
AVhen motion picture film leaves the factory it may be reasonably assumed that it is free from latent .static since it is handled during manufacture with extreme care and under the most ideal conditions of humidit}^ Moreover, careful tests are made on the finished perforated film before shipment in order to insure that the film is free from latent static markings which might otherwise appear on the developed film.
During handling, static may be produced either in the camera or in the laboratory when winding the film onto racks, when processing on the developing machiDe, or during printing as follows:
Camera Static
Negative motion picture film when packed for shipment contains such a quantity of moisture that it is in equilibrium with an atmosphere of 70% to 75% relative humidity, and in this condition, and especially in the case of negative film, unless it is subjected to severe friction, no static trouble need be feared in practice.
In order to determine at what point or points in a camera static is usually generated, a roll of positive motion picture film was thoroughly desiccated over sulphuric acid and then passed rapidly through a camera in the dark. Static discharges were observed at the following points: (a) where the film parted from the spool at a tangent, (b) at the retort traps, (c) in the region of all sprockets even though grounded, (d) at the gate, (e) at the take-up roll.
So-called grounded collectors consisting of tinsel and graphite coated pads were placed against or near the film at two or three places but these had very little effect in preventing the static discharges. On development of the film the quantity of static markings ran parallel with the quantity of discharges observed in the dark.
This experiment demonstrated that static may be produced in the camera at any point where there is friction and especially in the camera gate where the discharges were most severe. It was also concluded that brush collectors are of questionable value, while a grounded metal crank is of little use unless the handle is in electrical connection with other parts of the camera such as the gate and sprockets, which must be good conductors.