Theory of magnetographic printing

The magnetic force of attraction between a recording surface and magnetic particles (the "developer" or "toner") is analyzed. The recording medium is assumed to be magnetically hard, the toner particles to be magnetically soft. The distribution of recording magnetization is taken to be periodic in the interior of an image area and uniform in an image free area. The toner particles are assumed to be part magnetic, part nonmagnetic material. In most of the calculations it is assumed that the magnetic susceptibility of the toner particles is small compared to unity. In the interior of an image area the force density has primarily a component perpendicular to the recording plane. This component decreases exponentially with distance from the recording plane, if the distribution of magnetization is sinusoidal. Near the edge of an image area the force density also has a tangential component, but this is generally smaller than the normal component. For toner particles with similar internal structure the force per unit mass always decreases with increasing particle size. Considered as a function of recording wavelength (at constant particle size) it shows a maximum at a wavelength comparable to the particle diameter.The force acting on a small particle (
\simeq 10\mu
m) in the immediate vicinity of the recording surface can be several hundred times the force of gravity. The implications of these results in regard to color printing are discussed.