1) Woodworking tool 
A stud finder (also stud detector or stud sensor) is a handheld device used to determine the location of wood and metal framing studs used in light-frame construction after the walling surface has been installed. There are two main types of these devices: magnetic stud detectors and internal capacitor stud finders. ==Magnetic stud detectors== Stat...
[n] - a small permanent magnet in a metal container
noun a small permanent magnet in a metal container; when the magnet clicks against the container it indicates that the magnet is directly over an iron nail that holds the wallboard to a stud