Abstract: An improved magnetic device for subjecting a liquid flowing through a pipe to a magnetic field in order to reduce subsequent crystalization of inorganic compounds. The device may be installed very easily and without first severing a pipe of an existing installation and provides a homogeneous magnetic field of high flux density. It has a housing with a central pipe-receiving passage and a slot extending from the passage through the periphery so that the pipe may be positioned in its passage. A plurality of magnets are contiguously arranged within the housing to surround most of the pipe except at the slot. A keyway is formed at the slot so that a key magnet body may be slid into the housing parallel to the pipe.