Static Fields

3. What are the sources of static magnetic fields?

Compass detecting the Earth’s magnetic field

The Earth’s static magnetic field is attributed to electric currents in the Earth’s core. Its strength varies at the surface between about 0.035–0.070 mT. Certain animal species make use of this field for orientation and migration.

Man-made static magnetic field are generated wherever electricity is used in the form of direct current (DC), such as in some transportation systems powered by electricity, in industrial processes such as aluminium production and in gas welding. These man-made fields can be more than 1000 times stronger than the Earth’s natural magnetic field (up to 60 mT).

The invention of superconductors in the 1970s and 1980s allowed for the use of magnetic fields more than 100 000 times stronger than the Earth’s natural magnetic field (up to 9400 mT).

Such fields are used in nuclear magnetic resonance (NMR) techniques which are the basis for medical imaging technologies such as magnetic resonance imaging (MRI) and spectroscopy (MRS).

MRI scanner (3000 mT)
Source: McKnight Brain Institute, U of Florida

Magnetic Resonance Imaging (MRI) provides three-dimensional images of soft body tissue such as the brain and the spinal cord. So far an estimated 200 million MRI scans have been performed worldwide.

In a clinical setting, MRI scanners routinely use static magnetic fields in the range of 200–3000 mT. These fields are generated by permanent magnets, by the flow of direct current (DC) through superconductors , and by combinations of the two. In medical research, fields of up to 9400 mT are used to scan the entire body of patients.

Operators of MRI scanners can be exposed to significant magnetic fields; exposure levels at the console are typically about 0.5 mT, but may be higher. Workers can be exposed to fields exceeding 1000 mT during the construction and testing of these devices, or during medical procedures when MRI scans a performed before, during, and after an intervention such as the removal of a brain tumour (interventional MRI).

Moreover, several physics research facilities and high-energy technologies employ superconductors. As a result workers can be exposed regularly and for long periods to fields as high as 1500 mT. More...