An electrostatic detection device, or EDD, is a specialized piece of equipment commonly used in questioned document examination to reveal indentations or impressions in paper that may otherwise go unnoticed. It is a non-destructive technique (will not damage the evidence in question), allowing further tests to be carried out. It is a sensitive technique capable of detecting indentations on pages several layers below the top sheet and many years after the indentations were created. EDD equipment and investigative techniques were central to overturning a number of convictions in the United Kingdom, as it was possible to demonstrate that witness statements had been altered or signed as blank pages in reverse order to the main notes. This was central in a number of cases investigated at the West Midlands Serious Crime Squad that were appealed. The allegations of tampering with evidence and witness statements led to the unit being disbanded, and over 60 convictions being quashed, many of the appeals relying on EDD evidence. How it works When writing is fashioned on a sheet of paper resting upon other pages, the indentations or impressions produced are transferred to those below. These transferred impressions can be detected using an EDD. In some situations, a questioned document such as a ransom note, or an extortion letter, may exist which can be determined to be the source of indentations detected on another piece of paper (e.g., an offender's notepad). Alternatively, indentations detected on a business contract might match information present on another such document. In some situations this would be an entirely innocent finding; however, if the two businesses are supposed to be operating independent of one another, then the finding could be significant. Decipherable indentations may also provide valuable information even when a second document is not present or cannot be located. For example, an anonymous letter may bear impressions of writing that relate to some mundane activity of the offender which could ultimately lead an investigator to a particular suspect. The electro-physical basis whereby an EDD actually works is complex. The original theory suggested that the paper sandwiched between the grounded platen and the mylar charging film acted as a type of capacitor with the change in capacitance being due to differing compression of the paper. This led to models like the 'Thickness Variation Theory' and the 'Surface Variation Theory'. However, it turns out that a detectable 'indentation' is not due to