A magnetometer is built-in since the iPhone 3GS, which is used to measure the strength and direction of the magnetic field in the vicinity of the device. Sometimes certain devices or radio signals can interfere with the magnetometer requiring users to either move away from the interference or re-calibrate by moving the device in a figure-eight motion. Since the iPhone 3GS, the iPhone also features a Compass app, which was unique at time of release, showing a compass that points in the direction of the magnetic field.