In this paper, we propose an experiment for analysing harmonic motion using an iPhone's (or iPad's) magnetometer. This experiment consists of the detection of magnetic field variations obtained from an iPhone's magnetometer sensor. A graph of harmonic motion is directly displayed on the iPhone's screen using the "Sensor Kinetics" application. Data from this application was analysed with "Eureqa" software to establish the equation of the harmonic motion. Analyses show that the use of an iPhone's magnetometer to analyse harmonic motion is a practical and effective method for small oscillations and frequencies less than 15-20 Hz.