Magnetic field cancelling systems provide an effective solution for dampening ambient magnetic fields, allowing high resolution performance of TEM, SEM and FEG SEM instruments. The system comprises of a magnetic field control unit, magnetic field sensor and three multicore cables. The cables are installed in the room on the X, Y and Z axes. The sensor measures the strength and phase of the field in real time. The signals are fed to the control unit where three power amplifiers drive anti-phase currents through the multicore cables to effectively nullify the field.