Olefinically unsaturated hydrocarbons are prepared from corresponding paraffinic hydrocarbons, in particular propylene is prepared from propane, by dehydrogenation over a catalyst comprising an oxide of a transition metal of group IV B of the Periodic Table, eg. The lower surface of the hull body is divided into a fore portion, a central portion and an aft portion, with a central recess being formed in the central portion and an aft recess being formed in the aft portion. The first and second inputoutput devices are operatively connected to the controller via a second communication bus. The magnetic field sensor is used to detect a change in magnetic field caused by the device moving relative to the hearing aid.