Since stationary magnetic stud detectors rely on the user to feel the attraction they can be unreliable, especially when the metal fasteners are located more deeply in the walling, which decreases the overall attraction. When fasteners are deep or buried under thicker walling material (as in most plaster walls) the effectiveness of stationary magnets is considerably lower. Moving magnetic detectors Moving magnet stud detectors use a neodymium magnet that is suspended in an enclosure and free to move in response to hidden metal. The strength of this rare earth magnet and the easy movement of the magnet allow moving magnetic stud detectors to work on a broad range of construction types. The magnet is suspended in such a way that it always sits in its "home" position until it is moved directly over a metal fastener or metal stud. On walls with shallow fasteners, the magnet moves towards the wall with such velocity that it makes a distinct thud sound when it hits the wall. Because a moving magnet is not dependent on the operator to feel the attraction to the metal, fasteners buried more deeply under plaster or tile can be located with this detector. For deeper fasteners, the sound is softer since the speed of movement is slower. Electronic stud finders Electronic stud finders rely on sensors that detect changes in the dielectric constant of the wall. The dielectric constant changes when the sensor is over a stud. The lower reading indicates the presence of a stud in the wall. Internal capacitor stud finders can also come with other features that locate metal and live AC voltage. Electronic stud finders currently come in three types: edge finders, center finders, and instant finders. Edge finders Edge finders are the most basic internal capacitor detectors. Edge finders detect the edges of the stud or other material behind the walling. This finder must first be calibrated over an empty section of the wall, and then it can be moved along the wall until it senses a change in density - such as the edge of a stud. Edge finders should be moved from both directions to find both edges of the stud. The single sensor in edge finders can be prone to error, sometimes indicating a spot 25 mm (1 in) or more from the stud's edge. Once both edges have been marked, the user must determine the location of the