Abstract: Apparatus is provided for performing the purity adjustment in slotted mask color picture tubes of the type having in-line electron guns located in side-by-side relationship along a single plane. The apparatus includes first and second bar magnets located externally of the tube and parallel to the gun-containing plane. Each of the bar magnets is oriented to generate a magnetic field in substantially the same plane, that plane being perpendicular to the gun-containing plane. Means are provided for jointly moving the bar magnets between a first position wherein the net resultant field generated across the gun containing plane is zero, and second position wherein the net resultant field generated by the bar magnets across the gun-containing plane is greater than zero and has a substantially equal effect on each of the guns. In a first embodiment, the moving means comprises a support upon which each of the bar magnets is fixedly mounted.