Magnetic compensation systems, also called magnetic cancellation or active compensation systems, were especially developed to reduce low frequency magnetic stray fields. With those compensation- / cancellation systems, electron microscopes, electron beam lithography and other tools, based on electron- or focused ion beams are safely protected from those stray fields.
Compensation systems typically reduce lower frequency magnetic fields in frequency range between 0.01 Hz up to 1000 Hz. However, some systems are able to cancel higher frequencies, too. Those magnetic fields are typically caused by moving cars, tramways, elevators, sweeping magnets, railways, electrical installations like transformers, switch gear, bus bars and overhead lines.
Compensation systems work best in homogeneous fields with low gradients. This applies typically to remote sources like tramway, railway and overhead lines, meaning, cancellation of the fields of local sources such as bus bars, cables and transformers is somewhat limited.