This app will help you to detect ferrous metal objects.

It uses the magnetometer to measure the magnetic field when your iPhone is placed near any metal objects. Calibrate your device’s magnetic sensor by moving the device in a "Figure 8" pattern and you can quickly detect wood studs hidden under walls, drywall, plasterboard or stucco.

Disclaimer: The performance of the app is limited by the construction of the wall, presence of a phone cover and/or the material of the nails in the studs.

Instructions: If your device has an external case then please remove it before using this app.