From 'forensic science' Andrew R.W. Jackson and Julie M. Jackson. Publisher: Pearson prentice hall.
ESDA which stands for Electrostatic detection apparatus, is a machine used within forensics for document examination. The machine allows the visulisation of indented writing wihout damaging the document.
The questioned document (usually a piece of paper) is put onto the bronze plate, then a thin film is placed over it. A corona (a highly charged wire) then passes over the film, giving it an electrostatic charge. The areas in which the paper is indented will have the greatest charge. Once the corona has passed over the film, the bronze plate is titled and photocopy toner powder (with small glass beads) is added. The toner then adds to the areas of greatest charge, and thus allows the indentation to now be seen.