A stud finder (also stud detector or stud sensor) is a handheld device used with wood buildings to locate framing studs located behind the final walling surface, usually drywall. While there are many different stud finders available, most fall into two main categories: magnetic stud detectors and electric stud finders. There are also some devices employing radar. History Stud finders have been in use since the early 20th century, and the first ones were all magnetic, relying on internal magnets to detect the walling fasteners or nails presumably attached to studs. In 1977, Robert Franklin designed an electronic stud finder that relied on an internal capacitor to measure changes in density behind the walling. His patent was put into production by the Zircon Corporation, which became the sole producer of electronic stud finders until the patent expired in 1998. While novel, these electronic stud finders did not always prove effective in locating studs. Since 1998, many developments and improvements have been made to the internal capacitor stud finders and increased their popularity. Recent developments include stud finders with multiple sensor plates that sense the wall in multiple places. These sensors can indicate the location, width, and lack of studs simultaneously. With more sensors, these stud finders do not require calibration and adapt better to inconsistencies in wall construction. As of 2013, a few stud finders were available on the market based on ultra-wideband radar scanners for sensors. They are based on the micropower impulse radar as the underlying technology, and stud finders using this technology was invented by Thomas McEwan and patented in 1995. Magnetic stud detectors Magnetic stud detectors use magnets to locate metal in the walling material because the magnet is attracted to the metal. The attraction grows stronger as the magnet gets closer to the metal in the walling. The strongest attraction point, if due to a metal fastener in the wall, should indicate the location of a stud. Magnetic stud detectors may be less useful in homes built with metal mesh lath and plaster. The metal mesh will confuse the signal of an electronic stud finder. Stationary magnetic detectors Stationary magnet stud detectors use a small stationary magnet to detect the nails or screws placed into studs while putting up the walling material. The user must move the magnet around the wall until feeling the pull of magnetic attraction, and move in the direction of attraction.