A search coil magnetometer has wire wrapped in coils around a ferromagnetic core. When the coil passes through a magnetic field, the field induces voltage in the coil. As the magnetic field fluctuates, so does the voltage it induces. By measuring the voltage over time, a search coil magnetometer can track changes in magnetic fields. To capture rapid fluctuations, these magnetometers can sample fields at a rate of two million times per second!