barrier between the platen and questioned document. Books and other multipage items can also be examined in some instances but these require additional preparation. Ideally, individual pages should be removed from such items but this is not always possible. As an alternative, a sheet of conductive material (e.g., aluminum foil) can be used to contact the conductive surface. Covering with charging film: As noted above, the charging film should cover the document completely and contact the platen all the way around the document. The charging film serves two purposes: it protects the document from subsequent toner application and it takes the electrostatic charge required to 'develop' any latent indentations on the document. The film should be laid over the document taking care to avoid wrinkles or excessive stretching of the film. Most EDD devices have a roll of bulk film positioned adjacent to the device for ease in application. The charging film will adhere to the platen due to suction applied through the surface of the platen. Charging of surface: the top surface of the charging film must be electrostatically charged. To achieve this most EDDs use a shielded (grounded) hand-held unit containing a high-tension corona wire which is usually about 7kV. It is turned on and literally waved over the entire surface of platen for several seconds criss-crossing the platen area. The objective is to produce an evenly distributed electrostatic charge in the air above the surface of the charging film. After passing the unit over the platen, the examiner waits for a few minutes until the charge has been passed to the surface of the charging film. Application of toner to develop indentations. The EDD is able to visualize indentations because the surface is differentially charged depending upon whether or not an indentation is present. The negatively charged toner particles are attracted to areas where there are indentations in the paper surface. Once an electrostatic charge has been applied to the charging film, the latent indentations can be developed. This is done using a powder toner very much like toner found in any electrophotographic printing device. There are various methods used for this purpose. Cascade method: Some EDDs have a platen that is hinged on one edge to permit it to be tilted. A mixture of toner and small carrier beads (made of glass or similar non-conductive material) is then poured over the surface of the document from