Secondary impressions of writing detected by ESDA

Experiments have been performed which show conclusively that, under certain conditions, ‘impressions’ of writing can be detected by ESDA (ElectroStatic Detection Apparatus) on paper which has been placed in contact with the reverse of paper bearing writing which has significantly embossed the back. Such indirect ‘secondary’ impressions are caused by some degree of lateral relative motion of the two sheets of paper. Secondary impressions can usually be distinguished from normal primary impressions (caused by the ‘act’ of writing) by several characteristics, in particular, by the indistinct, incomplete and, sometimes, diffuse nature of the secondary impression images.