A magnetic field indicator is provided which visually indicates the presence of a potentially harmful magnetic field by means of a magnetically responsive additive. In one embodiment, the additive comprises ferrite-encapsulating microspheres which contain ferrite flakes. The ferrite flakes are movably suspended within the microspheres so that the flakes are free to align with the lines of flux of a magnetic field. In the presence of a magnetic field, the ferrite flakes align themselves with the lines of flux of the magnetic field, thereby changing the reflectivity and other visual characteristics of the material which incorporates the ferrite flakes, so that an observer is able to visually detect the presence of a magnetic field. The magnetic field indicator may be embodied in accordance with a variety of applications. These embodiments include a coating, a solid, a gel, and a cloth, each of which may be affixed to, or incorporated within, the housing of a magnetic storage device, so that a visual indication of the presence of a potentially damaging magnetic field is provided. In a preferred embodiment of the invention, a pattern is magnetically written on the surface of a material which incorporates the magnetically responsive additive. This pattern provides a higher contrast between the activated state (that state in which a magnetic field is detected) and the unactivated state of the magnetic field indicator device. The present invention may also be employed as a display means for electro-magnetically controlling visual displays, polarizing windows, and shutters.

In situations such as the checkout of library books and video cassettes issued by video rental stores, it is desirable to indicate the date by which these materials are due. This patent describes a method and means for displaying the due date by using an erasable and rewritable media affixed to the issuable material. Device for writing/erasing this media is described. Furthermore, method and device for simultaneous reading of bar codes, writing the due date and altering the detectability state of a programmable magnetic strip are disclosed.

An apparatus for obtaining carrier particles which comprises in operative relationship a container means, a rotating means, a moving transporting means in contact with the container means and the rotating means, and a magnet means attached to the transporting means.

The microcapsule magnetic display sheet is sequentially provided thereon with a transparent front sheet, a microcapsule layer coated on the transparent front sheet and then a ultraviolet ray curable resin layer. The ultraviolet ray curable resin layer is formed by painting liquid ultraviolet raycurable resin on the microcapsule layer first and then hardening it by irradiating it with UV rays. As the liquid ultraviolet ray curable resin can fill fine gaps of the microcapsules, no hollows left between the microcapsules and the UV resin layer. As the UV resin filled in gaps shares wording pressure, the microcapsules are fully protected and hardly be destroyed. After painting the liquid UV ray curable resin on the microcapsule layer and before it IS hardened, a separable film is covered on it. While a film is covered on it, the sheet is hard and if the film is separated, the sheet is soft. So the stiffness of the magnetic display sheet can be regulated by covering or not covering a film.

A stable, non-agglomerating, magnetic recording coating ink, for use in coating a flexible non-magnetic substrate, is made by a two-step process which first mixes finely divided chromium dioxide particles with water and a polymeric surfactant/dispersant agent which has as its major functionality in a side chain, carboxylic acid, as a free acid, ester or amide thereof, and of a relatively low number average molecular weight in the range 500 to 20,000. Only after the particles are dispersed in this slurry is water-based polyurethane binder added to the slurry, as the second step in the process, to form the final ink.