The Model 5451 Electromagnet is a Coil pair arranged in approximately Helmholtz geometry. It has no magnetic shielding and hence the magnetic field within the central volume is the vector sum of the field created by the Helmholtz Coil and the environmental magnetic fields at the same location. Environmental magnetic fields arise from the Earth’s geomagnetic field (typically about 0.5mT or 0.5G), from magnetic materials like steel, from permanent magnets, from electric currents and from eddy currents in electrically conducting materials exposed to a time varying magnetic field. To minimize the effects of environmental magnetic fields the first approach is where feasible, keep magnetic sources well away (approximately 1m or greater) from the center of the 5451. To accurately determine the magnetic field within the working volume it is necessary to map the actual field as a function of position using an appropriate magnetic sensor such as the Bartington Mag-03 Three Axis Magnetic Field Sensor or the Senis CH3A Three Axis Magnetic Field Transducer.
For more information please contact Ian Walker for information on this subject.