The electromagnetic pickup of an electric guitar is a nonlinear device that provides a pleasant distortion. Although the pickup is a simple device consisting of a coil and a few magnets or pole pieces, the measurement of its nonlinear function is a difficult task. This paper shows a measurement technique that can estimate the nonlinear function of a pickup in both y− and z− directions of the vibrating string. The experimental results are provided for three different types of pickups: a single-coil pickup with six staggered pole-pieces, a humbucker pickup with six equal height pole-pieces, and a humbucker rail pickup. The measured non-linear functions of the three pickups are very different from each other, leading to different distortions. These experimental results confirm that the pickup geometry plays an important role in distortion.