The apparatus for the visual display of electrical signals recorded on a magnetic medium or support is in the form of a "magnifying glass" which is a plate or sheet of transparent material in which there is a dimple closed by a membrane. The volume of the dimple is filled by a mild ferrite dispersion...http://www.google.se/patents/US4345820?utm_source=gb-gplus-sharePatent US4345820 - Visual display apparatus for electrical signals recorded on a magnetic medium
