Abstract: A plurality of magnets are detachably attached to the exterior of the cylindrical casing of a liquid filter unit to attract and capture ferromagnetic particles from the fluid, particles too small to be captured by the basic filtering element but large enough to cause further harm if allowed to remain free in the liquid. The magnets are magnetized such that their ends are north or south poles and shaped so that the ends are in close, positive contact with the casing. The attachment apparatus comprises springs interconnecting the magents and links and springs to position the magnets in the lengthwise direction of the casing.