Invisible writing unit for temporarily recording information - provides writing surface of magnetisable plate or foil reacting to writing implement with magnet

摘要

The writing unit provides a writing element or instrument, a writing surface and selective appts. for making visible the information written on the surface. A magnetisable plate or foil is used for the writing surface and the writing element has a magnet with a remanence and coercive field strength at least equal to that of the magnetic plate or foil. A sensor film has pockets of power in a wet state or dispersed in a fluid that can react with a magnetic field to render the information visible. This film is above the writing surface. USE/ADVANTAGE - Security device, safe to use, easy to service and light to handle.