Abstract: A stylus having an electrically-conductive writing tip is used to write on an electrically-sensitive paper, for example a metallized paper in which the metallization is eroded by the current passed by the stylus. The stylus is energized by a coded train of electrical pulses whereby the coding is reproduced in the writing which can thus be verified as genuine. The material to be written on, for example an airline ticket whose fraudulent alteration is to be exposed, may be supported on a stand having a conductive resilient arm bearing on the electrically sensitive surface to provide a return path for the stylus current. The principle can be applied with coded optical pulses directed onto an optically-sensitive paper.