Abstract: A device for validating the authenticity of a document uses changes in capacitance due to the presence of an electrically conductive security thread extending across the document. The device uses a drive arrangement for moving of a document such as a bank note, past a sensing arrangement which senses changes in capacitance due to movement of a security thread therepast. A frequency generator produces a high frequency time varying oscillator signal and provides this signal to an elongate oscillator electrode extending across the path of movement. A lead elongate measuring electrode is electrically conductive and positioned in front of the oscillator electrode and extends across said path and a trailing elongate measuring electrode is positioned behind the oscillator electrode and extends across the path. The signals from the measuring and the oscillator electrode are provided to a signal processing arrangement.