Photos by Luke Abaffy for ENR The iPhone beeps when its magnetometer, in the upper right of the phone, is near metal. Related Links: Stud Find App on the iTunes Store Stud Find is an iPhone application that uses the device's built-in magnetometer to find metal studs, screws, nails and anything metallic in a wall.The iPhone's three-axis magnetometer is an instrument that measures magnetic forces. Internal Apple software uses the instrument to position the phone's 360° orientation. Stud Find uses the magnetometer as a metal detector.Testing the app on an iPhone 5, we found it works best when moved slowly