when no magnetic field is applied removing magnetically reactive medium 105 from gap 106 when no magnetic field is applied decreases friction thereby reducing drag between first rotatable member 70 and second rotatable member 80 while the illustrative magnetic particle clutch 54 is described as having features 78 and 96 it is not intended to be so limited accordingly first and second rotatable members 70 80 may contain no non magnetic features when additional working surface area is not required clutch 54 further includes a source of magnetic flux which is depicted in fig3 and 5 as an electromagnet 110 mounted on the outside of housing member 52 between first rotatable member 70 and housing member 52 in the illustrated embodiment electromagnet 110 comprises a wire wound coil 112 surrounded by a generally toroidal shell 114 as is well known an electrical current applied to coil 112 generates a magnetic field in the vicinity of electromagnet 110 the intensity of which is proportional to the level of current provided alternatively the source of magnetic flux may comprise other arrangements such as for example a permanent magnet supplemented by a counteracting electromagnet so that clutch 54 will default to being engaged should the electromagnet fail electromagnet 110 is controlled by an electronic control unit ecu not illustrated that provides an electrical signal to coil 112 via wires 115 the ecu processes input such as for example sensor readings corresponding to vehicle parameters and processes the input according to logic rules to determine the appropriate electrical signal to provide electromagnet 110 the ecu is preferably microprocessor.