Photos by Luke Abaffy for ENR
The iPhone beeps when its magnetometer, in the upper right of the phone, is near metal.

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 along the wall being searched, with the right side of the iPhone held perpendicular to the surface—that's because the magnetometer is located in the upper-right corner of the phone. When the magnetometer finds metal behind the drywall, the phone beeps and a light-bulb image on the screen glows. As the magnetometer gets closer to the metallic source, a bar under the light bulb goes from green to yellow to red (see photos). When fully red, the metallic object should be behind the phone's upper-right corner. In tests, ENR found that if the bar gets stuck in a position or acts erratically, holding it away from anything metal will reset the magnetometer.

Stud Find has three stars out of five in the Mac App Store. However, many of the negative reviews suggest users did not follow the directions or failed to realize the app detects only metal studs.