This paper first presents the structures of permanent-magnet pickups for electric guitar and the considered device: string, magnet, coil. It then describes a method to calculate the induced electromotive force (EMF) in the pickup coil when the string moves. The method of calculation links the EMF in the pickup coil with the flux cut by the string when it moves. The EMF is of course nonlinear. The harmonics of the EMF can be calculated. This is a first step towards the final aim that will consist in studying the frequency spectrum of the electrical signal given by the pickup, which is quite complicated, in order to link the magnetic structure and the string movement with the musical effect. Analytical calculations using the Coulombian model of magnets are used to evaluate the magnetic field created by the magnet and the electromotive force in the pickup coil.