Method of dividing a sintered oxidic ferromagnetic ring core and a deflection unit

A method of dividing a ring core of sintered oxidic ferromagnetic material for a deflection unit in two semi-annular parts, which outside diameter of which ring core has different dimensions, measured in different planes extending perpendicularly to its longitudinal axis, the ring core being parted along two dividing seams, characterized in that each division seam is formed by means of a spotshaped heat source which supplies heat locally to the surface of an end of the ring core to form thermally induced stress areas, and which is moved relative to the ring core along a line substantially in the direction of the longitudinal axis of the ring core, which line defines a division seam, such that the thermally induced stress areas are moved across the ring core, a value being used for the ratio between the heat supplied and the relative velocity with which the heat source is moved such that the ring core parts spontaneously and in a controlled manner along the division seam due to cracking caused by the thermally induced stress areas.