Magnetography

(or ferrography), a method for printing letters, numbers, and other images on ordinary paper using magnetic powder. Magnetography is most frequently performed using an intermediate magnetic recording medium.

In some devices the intermediate medium is a magnetic drum, along whose circumference are located the magnetic recording heads, developing section, contact roller, cleaning unit, and erase head. The magnetic drum revolves evenly during the operational cycle of the device. A latent magnetic image of the symbol is produced in the drum’s magnetic layer in the form of a mosaic of separate magnetic imprints generated by the magnetic recording heads. In the developing section, particles of a ferromagnetic powder are attracted to the magnetized portions of the drum surface, forming a visible image of the recorded symbols. Upon contact with the paper, the magnetic powder sticks to its surface. In the simplest case, the resulting imprints are fixed by pressing the particles of powder into the paper surface by compression between rollers. To achieve better adhesion to the paper, the ferromagnetic material is coated with a thermoplastic resin and the rollers are heated. As the paper passes between the rollers, the resin melts and bonds the powder image firmly to the paper. Any powder remaining on the drum after the image has been transferred onto the paper is removed by the cleaning unit by fur brushes and a stream of air; the latent image is erased by the magnetic erase head and the drum is ready for a new recording. When several copies are required, the latent magnetic images are not erased and the printing process may be repeated for a virtually unlimited number of times.

The minimum image size of a symbol produced by magnetography is 2 ×3 mm. The speed of printing on the apparatus described above is usually 6,000 lines per min, but it may be increased considerably. The main application of magnetography is in printers for electronic computer printouts.