Electric guitar pickup (electromagnetic induction)

String effect on a single coil (electric guitar pickup such as those on Stratocaster). The coil is connected to a multimeter that indicates the voltage changes when the string is subject to vibration. This signal is normally sent to the amplifier.

Warning:  the voltage should reach its maximum value when the string is at maximum velocity, and not when it is at maximum displacement. 

Source:  Wikipedia (animation made by Dake)