Abstract: An apparatus and method for sensing the swelling and shrinking of an elastic member based on movement and the angular changes to the surface of the elastic member between one or more portions of the surface thereof. The apparatus includes first and second parts which cooperate to produce an electrical signal indicative of the angular and distal positioning of the respective parts, with one of the parts preferably being a Hall effect device and the other a magnet. At least one part is mounted on a flexible base which attaches to a first portion of the surface of the elastic member for movement therewith with the other mounted for movement with a second surface portion of the elastic member. The apparatus and method are particularly adapted for use on a male penis and female breast for estimating sexual arousal based on the swelling of such body parts due to engorgement with blood which accompanies sexual arousal.