Abstract: The invention relates to a method and a device, in particular for treating a fluid, having at least one multipart housing 1 and at least one permanent magnet 36 which can be mounted on a support body 35, the housing 1 extending at least partly around the external periphery of the support body 35, and adaptor elements, comprising individual magnet supports 22 and orientation retainers 21, for holding a freely selectable number of permanent magnets 36 in the axial direction of the support body 35 are provided with variable overall size. As a result of this, a number of magnets 36 can be selected as a function of the fluid composition and fitted into the housing 1, so that individual matching and optimum action of the magnetic field on the fluid is obtained.