What Are Magnetic Fields?

Magnetic fields are vector fields that describe the influence exerted by moving electric charges and magnetic dipoles on their surroundings. They arise wherever electric currents flow, whether in a wire carrying direct current, in a plasma orbiting a planet, or in the spinning and orbiting electrons within atoms. The SI unit of magnetic flux density, the tesla (T), honors the engineer Nikola Tesla; the related quantity, magnetic field strength, is expressed in amperes per meter (A/m). Magnetic fields are inseparable from electric fields: any change in one produces the other, a coupling that underpins all electromagnetic phenomena from radio waves to light.

The discipline draws on classical electromagnetism, quantum mechanics, and materials science. Engineering applications range from motors and transformers to medical imaging and transportation systems, while basic research uses magnetic fields as a probe for the structure of matter at atomic and subatomic scales.

Governing Physics

The behavior of magnetic fields is governed by two of Maxwell's four equations. Gauss's law for magnetism states that magnetic field lines form closed loops with no isolated magnetic monopole sources, a fact that distinguishes them from electric fields. Ampere's law, as extended by Maxwell's displacement current term, relates the circulation of the magnetic field around a closed path to the enclosed current and the rate of change of the electric flux. Together, these equations predict electromagnetic waves and define how fields propagate through space and matter. The historical development of Maxwell's equations, from Maxwell's 1864 formulation through Heaviside's 1884 simplification to Hertz's 1888 experimental confirmation, established the theoretical foundation of modern electrical engineering.

Material Response

When a magnetic field passes through matter, the response depends on the material's magnetic constitution. Paramagnetic materials, such as aluminum, align weakly with an applied field. Ferromagnetic materials, including iron, nickel, and cobalt, undergo strong alignment of magnetic domains and retain magnetization after the field is removed; the residual flux density is called remanence, and the field required to demagnetize the material is its coercivity. Saturation magnetization is the maximum flux density a ferromagnetic material can sustain, a property that governs the design of transformer cores and permanent magnets. Diamagnetic materials, by contrast, weakly oppose applied fields. Superconductors exhibit perfect diamagnetism below their critical temperature, completely expelling flux in what is called the Meissner effect.

Detection and Measurement

Measuring magnetic fields requires instruments matched to the field strength and frequency of interest. Hall effect sensors and fluxgate magnetometers cover the range from nanotesla to several tesla and serve industrial and navigation applications. At the femtotesla level, NIST's chip-scale atomic magnetometer research demonstrates optically pumped sensors capable of detecting the faint magnetic fields produced by neural activity without cryogenic cooling. SQUID magnetometers, operated near absolute zero, remain the reference standard for the most demanding biomedical and metrology applications, achieving sensitivities below 1 fT/sqrt(Hz). The NIST magnetic and electric fields program maintains calibration facilities traceable to SI units across a wide span of field strengths.

Applications

Magnetic fields have applications in a wide range of disciplines, including:

• Electric motors, generators, and power transformers in energy conversion and distribution
• Magnetic resonance imaging (MRI) for medical diagnostics, requiring highly uniform fields above 1 T
• Compasses and inertial navigation systems exploiting Earth's geomagnetic field
• Magnetic anomaly detection for locating submarines, buried pipelines, and unexploded ordnance
• Magnetic levitation vehicles that use controlled repulsion forces to eliminate mechanical contact
• Biomagnetics research into the magnetic properties of biological tissues and organisms