What a magnetic field is
Sprinkle iron filings on paper over a bar magnet and they form curves running from the north pole to the south pole: these are field lines, showing the direction of the magnetic field. Fields surround not only magnets but any wire carrying a current, and a charge moving through a field feels a force at right angles to its motion (the Lorentz force). Permanent magnets are magnetic because tiny magnetic moments from electron spin and orbital motion line up in the same direction. Field strength is measured in teslas, and it varies enormously from place to place.
| Source | Field strength |
|---|---|
| Earth's surface | 25–65 µT |
| Fridge magnet | ~5 mT |
| MRI scanner | 1.5–3 T |
| Neutron star (magnetar) | 10⁹–10¹¹ T |
The birth of electromagnetism
In 1600 William Gilbert argued in De Magnete that the Earth itself is a giant magnet. In 1820 Hans Christian Ørsted noticed during a lecture that a compass needle moved next to a current-carrying wire, showing that electricity and magnetism are linked. In 1831 Michael Faraday discovered electromagnetic induction — a changing magnetic field produces a current — the principle behind generators and transformers. In 1865 James Clerk Maxwell unified it all in four equations and showed that light is an electromagnetic wave. In the 20th century the Meissner effect, in which superconductors expel magnetic fields, opened the way to magnetic levitation.
Earth's field and everyday magnetism
Earth's field is sustained by a geodynamo: convection of molten iron in the outer core generates electric currents. The field deflects charged particles from the solar wind and cosmic rays, protecting the atmosphere and life, while particles funnelled toward the poles create the aurora. The magnetic north pole drifts by tens of kilometres a year, and rocks record that the poles have swapped places — a geomagnetic reversal — roughly every few hundred thousand years, most recently about 780,000 years ago. Migratory birds and sea turtles are thought to navigate by sensing the field. Electric motors, loudspeakers, hard drives, induction hobs and MRI scanners all rely on magnetic fields.
FAQ
- Are magnetic fields harmful?
- Fields from household appliances and power lines are far below international exposure limits, and no health effect has been established. Strong fields such as MRI require screening for metal implants.
- What would happen if Earth's magnetic field disappeared?
- It will not vanish suddenly, but a much weaker field would leave satellites and power grids more exposed to space weather and, over very long times, let the solar wind strip away atmosphere.
Sources reviewed: 2026-10-26
References
- Gilbert, W. (1600). De Magnete. London: Peter Short.
- Maxwell, J. C. (1865). A dynamical theory of the electromagnetic field. Phil. Trans. R. Soc. Lond., 155, 459–512.
- Glatzmaier, G. A., & Roberts, P. H. (1995). A three-dimensional self-consistent computer simulation of a geomagnetic field reversal. Nature, 377, 203–209.
- Purcell, E. M., & Morin, D. J. (2013). Electricity and Magnetism (3rd ed.). Cambridge University Press.
- Ørsted, H. C. (1820). Experimenta circa effectum conflictus electrici in acum magneticam. Copenhagen.
- Faraday, M. (1832). Experimental researches in electricity. Phil. Trans. R. Soc. Lond., 122, 125–162.
- Meissner, W., & Ochsenfeld, R. (1933). Ein neuer Effekt bei Eintritt der Supraleitfähigkeit. Naturwissenschaften, 21, 787–788.
- Larmor, J. (1919). How could a rotating body such as the Sun become a magnet? Reports of the British Association, 159–160.
- Wiltschko, W., & Wiltschko, R. (2005). Magnetic orientation and magnetoreception in birds and other animals. Journal of Comparative Physiology A, 191, 675–693.