A stud finder is a handheld device used to determine the location of wood and metal framing stud

Wall stud

A wall stud is a vertical member in the light frame construction techniques called balloon framing and platform framing of a building's wall.-Purpose:...

s used in light-frame construction after the walling surface has been installed.

There are two main types of these devices:

• The first uses a small magnet
  Magnet
  A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
  
   to detect the nails or screws placed into studs during the manufacturing of the wall.
• The second type uses an internal capacitor
  Capacitor
  A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
  
   plate to detect changes in the dielectric constant
  Dielectric constant
  The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
  
  of the wall as the user moves the finder over the surface of the wall. A significant change in the dielectric constant indicates a dense object behind the wall, normally a framing stud. Internal capacitor stud finders are the most common.

Internal capacitor stud finders are divided into two classes:

• Edge finders detect the edges of the stud or other target behind the wall. The sensor in an edge finder detects when it's directly over a change in density, such as the edge of a stud. Edge finders have to be moved from both right and left directions to find both edges so the center can be determined. The first internal capacitor stud finders were all edge finders, and most inexpensive models still are.
• Center finders detect the center of the stud. These tools register the wall’s dielectric constant from multiple sensors, and use the different readings to determine the location of the target center. Unlike edge finders, these stud finders need to be moved from only one direction to indicate the center of the stud. These kinds of stud finders are also called "one step" finders.

Internal capacitor stud finders also can come with other features that locate metal and live AC voltage.

The development of the internal capacitor stud finder

The internal capacitor stud finder was first designed by Robert Franklin in 1977. After he had patented the device, he tried to sell the rights to produce it to several major tool companies, all of which turned down the idea. He took the design to Zircon Corporation, an electronics manufacturer in Campbell, California, who agreed to build and market the device. Zircon was the sole producer of internal capacitor stud finders until 1998, when the original patent expired, allowing other companies to manufacture it as well. Other developments and improvements in stud finders remain under patent protection.