Printer head for serial dot printer

Claim 1

In a printer head for a mosaic printing comprising of a first circular yoke plate; a cylindrical permanent magnet magnetized in an axial direction positioned on said first yoke plate; the permanent magnet having the characteristics that the flux generated by the permanent magnet decreases as the temperature increases; a ring shaped second yoke plate positioned on said permanent magnet; a plurality of electromagnets each having a column core and a coil wound on the column core, the electromagnets being positioned in a circle on said first yoke plate with predetermined angle intervals therebetween; an armature-print-needle assembly including a disk shaped circular resilient spring having a plurality of inwardly extending projections, a plurality of armatures each fixed on a corresponding projection of said spring and a plurality of print needles each fixed to a corresponding armature so that the print needle extends perpendicular to the plane of the spring; a guide frame covering said armature-print-needle assembly said guide frame having a thin linear slit for passing therethrough the tops of said print needles; wherein a first substantially closed magnetic path is formed from said permanent magnet through said second yoke plate, said armatures, said electromagnets and said first yoke plate to said permanent magnet such that an armature together with the corresponding print needle are attracted to the corresponding electromagnet by the magnetic flux circulating in said first magnetic path and said armature and the corresponding print needle are released by the force of the spring upon application of an electric current to the coil of the corresponding electromagnet to print a dot; the improvment wherein a cylindrical adjusting yoke means is coupled to said first yoke plate, said permanent magnet and said second yoke plate for forming a second closed magnetic path from said permanent magnet through the second yoke plate, the adjusting yoke means and the first yoke plate to the permanent magnet, said adjusting yoke means having a magnetic characteristic wherein the magnetic reluctance of said adjusting yoke means increases as the temperature of said adjusting yoke means increases.