Magnetic field
In 1600, William Gilbert published De Magnete and stated that Earth itself is a magnet. This work established magnetism as a science by mapping the invisible forces surrounding our planet. A magnetic field describes the influence on moving electric charges, currents, and materials like iron. When a charge moves through this field, it experiences a force perpendicular to its velocity and the field direction. Permanent magnets pull ferromagnetic materials while attracting or repelling other magnets. Nonuniform fields exert tiny forces on nonmagnetic substances through paramagnetism, diamagnetism, and antiferromagnetism. These effects are usually too small for human detection but measurable with laboratory equipment. The field surrounds magnetized objects, electric currents, and time-varying electric fields. Mathematically, scientists describe it as a vector function assigning a specific value to every point in space.
James Clerk Maxwell chose the letters B and H in his Treatise on Electricity and Magnetism volume two pages 236 to 237. He selected these symbols from the beginning of the alphabet for various quantities without deep symbolic meaning at first. Modern physicists debate whether B represents magnetic flux density or magnetic induction while H denotes field strength. In vacuum, the two fields relate through vacuum permeability denoted by mu naught. Inside magnetized materials, they differ by the material's own magnetization field. The International System unit for B is the tesla equal to newton per meter per ampere. The unit for H measures ampere per meter. Edward Purcell wrote in 1963 that calling B magnetic induction seems clumsy when asking physicists what curves pion trajectories in bubble chambers. Geophysicists rarely refer to Earth's magnetic induction while astrophysicists discuss galactic magnetic fields freely. Some textbooks now use magnetic field for both concepts despite historical reservations about terminology confusion.
French scholar Petrus Peregrinus de Maricourt mapped magnetic fields on spherical magnets using iron needles in 1269. He identified crossing points as poles and noted magnets always possess north and south ends regardless of size. William Gilbert replicated this work three centuries later and published De Magnete in 1600 claiming Earth acts as a giant magnet. John Michell stated magnetic poles follow an inverse square law in 1750 before Charles-Augustin de Coulomb verified it experimentally in 1785. Siméon Denis Poisson created the first successful model in 1824 describing fields produced by pole pairs. Three discoveries in 1820 challenged existing foundations when Hans Christian Ørsted showed current-carrying wires generate circular magnetic fields. André-Marie Ampère demonstrated parallel currents attract or repel depending on direction while Jean-Baptiste Biot and Félix Savart announced empirical force results. Michael Faraday discovered electromagnetic induction in 1831 showing changing magnetic fields create encircling electric fields. James Clerk Maxwell developed his equations between 1861 and 1865 explaining all classical electricity and magnetism together.
Scientists use instruments called magnetometers to measure local magnetic field intensity and direction. Important classes include induction magnetometers measuring only varying fields alongside rotating coil devices. Hall effect magnetometers detect charge separation perpendicular to current flow and applied magnetic fields. Nuclear magnetic resonance magnetometers utilize atomic spin properties for precise readings. Superconducting quantum interference device magnetometers achieve extreme sensitivity levels. Fluxgate magnetometers provide robust measurements for geological surveys. The finest precision attained came from Gravity Probe B reaching nanotesla scales. Electrons spiraling around field lines produce synchrotron radiation detectable as radio waves from distant astronomical objects. Iron filings placed near magnets form visible lines corresponding to field directions though they alter the actual field due to high permeability. Polar auroras display field lines visually when plasma particle interactions create light streaks aligned with Earth's magnetic field.
Rotating magnetic fields power alternating-current motors designed by Galileo Ferraris and Nikola Tesla independently in the late nineteenth century. Ferraris published research to the Royal Academy of Sciences in Turin in 1888 while Tesla gained patents for his electric motor in May 1888. Three-phase systems dominate global electrical supply because three coils at mutual angles of 120 degrees create stable rotating fields. Synchronous motors use direct voltage-fed rotor windings allowing controlled excitation while induction motors employ short-circuited rotors following stator fields. Short-circuited turns develop eddy currents induced by rotating fields producing torque through Lorentz forces on conductors. Magnetic circuits apply flux density inside linear materials similar to Ohm's law relationships between current density and electric field. Reluctance quantities calculate magnetic flux through complicated geometries using circuit theory techniques available since the mid-nineteenth century. The Hall effect measures dominant charge carriers as negative electrons or positive holes within semiconductor materials.
Convection of liquid iron alloy in Earth's outer core produces our planet's magnetic field through a dynamo process. Movements drive feedback loops where electric currents generate fields acting back upon those same currents. Surface field strength approximates that of a giant bar magnet tilted about 11 degrees off rotational axis. North poles of compass needles point toward the North Magnetic Pole which is actually the south pole of geomagnetic fields due to attraction rules. Field strength varies over time while locations shift periodically reversing orientation every few hundred thousand years. The most recent reversal occurred 780,000 years ago according to geological records. This shield protects Earth's ozone layer from damaging solar wind particles streaming outward from the sun. Without this protection atmospheric erosion would strip away vital gases needed for life support systems globally.
Albert Einstein published his special relativity paper establishing equivalence between electric and magnetic fields in 1905. Observers in different reference frames perceive electromagnetic forces differently depending on their relative motion states. Changing reference frames mixes components into rank-two tensors called electromagnetic stress-energy tensors combining energy stored in both fields. Quantum electrodynamics formalized notions that electromagnetic field energy exists as quantized photons rather than continuous waves. Virtual photons exchange between charged particles computed using perturbation theory produce Feynman diagrams representing interactions. Predictions match experiments to accuracy levels around one part in ten trillion limited only by measurement errors. Classical equations remain valid under everyday circumstances where quantum differences prove negligible for macroscopic objects. Retarded time solutions describe how influences travel at light speed across space from source to observation point.
Common questions
When did William Gilbert publish De Magnete and what was his main conclusion about Earth?
William Gilbert published De Magnete in 1600 and stated that Earth itself is a magnet. This work established magnetism as a science by mapping the invisible forces surrounding our planet.
What are the International System units for magnetic flux density B and field strength H?
The International System unit for B is the tesla equal to newton per meter per ampere. The unit for H measures ampere per meter.
Who mapped magnetic fields on spherical magnets using iron needles in 1269?
French scholar Petrus Peregrinus de Maricourt mapped magnetic fields on spherical magnets using iron needles in 1269. He identified crossing points as poles and noted magnets always possess north and south ends regardless of size.
When did Nikola Tesla gain patents for his electric motor and who else designed alternating-current motors independently?
Tesla gained patents for his electric motor in May 1888 while Galileo Ferraris published research to the Royal Academy of Sciences in Turin in 1888. Both scientists designed rotating magnetic fields power alternating-current motors independently in the late nineteenth century.
How long ago did the most recent geomagnetic reversal occur according to geological records?
The most recent reversal occurred 780,000 years ago according to geological records. Field strength varies over time while locations shift periodically reversing orientation every few hundred thousand years.