What Is Inductance?

Inductance is a property of an electrical conductor or circuit by which a change in current through the conductor induces an electromotive force in that conductor or in a neighboring conductor. Quantitatively, inductance is the ratio of the induced voltage to the rate of change of the current that causes it, expressed in henries (H) in the International System of Units. The concept traces to Michael Faraday's discovery of electromagnetic induction in 1831 and was formalized by Joseph Henry, after whom the unit is named. Inductance is a foundational concept in electrical engineering and physics, underpinning the design of transformers, inductors, motors, and radio-frequency circuits.

Self-Inductance and Mutual Inductance

Self-inductance describes the tendency of a conductor to oppose changes in its own current by generating a back-electromotive force proportional to the rate of change. When current flows through a coil, it establishes a magnetic flux through the interior of the coil; if the current changes, the flux changes, inducing a voltage that resists the change per Faraday's law. Mutual inductance describes an analogous coupling between two separate conductors: a changing current in one induces a voltage in the other. The OpenStax University Physics treatment of self-inductance and inductors provides a rigorous derivation from Faraday's law through the solenoid model. Mutual inductance is the operating principle of transformers and is also exploited in wireless power transfer systems and coupled-resonator filters.

Inductor Design and Core Materials

The inductance of a coil depends on its geometry and on the magnetic permeability of the surrounding material. For a simple solenoid, inductance scales with the square of the number of turns, the cross-sectional area of the core, and inversely with the length of the winding. Inserting a ferromagnetic core material such as silicon steel or ferrite increases the effective permeability and therefore the inductance for a given winding, but ferromagnetic cores introduce nonlinearity and core losses at high frequencies. Air-core inductors avoid these losses and are preferred in RF circuits operating above several megahertz. Toroidal geometries confine the magnetic flux within the core, reducing electromagnetic interference with adjacent components and improving energy efficiency. The mutual inductance treatment in OpenStax University Physics provides the analytical framework for computing inductance from first principles, relating coil geometry to the linked magnetic flux that determines the coupling coefficient. The choice of core material, winding configuration, and operating frequency are jointly optimized in practical inductor design, balancing inductance value, quality factor (Q), and current-handling capacity.

Measurement and Characterization

Inductance is measured using impedance bridges or LCR meters, which apply an AC signal at a known frequency and derive inductance from the measured reactance. NIST has published technical documentation on inductance measurement using LCR meters and current transformer interfaces, including calibration procedures traceable to primary impedance standards. The Maxwell-Wien bridge circuit, one of the classical null-detector bridge configurations, allows precise comparison of an unknown inductance against a calibrated capacitance standard. At high frequencies, parasitic capacitance of the winding causes self-resonance, above which the component behaves capacitively rather than inductively. Accurate characterization therefore requires measuring inductance at a frequency well below self-resonance and accounting for equivalent series resistance and skin-effect losses.

Applications

Inductance has applications across a wide range of engineering fields, including:

• Power supply design, including switch-mode converters and DC-DC regulators where inductors store and release energy each switching cycle
• Transformer design for AC power distribution and signal isolation
• Radio-frequency filters and matching networks in communication equipment
• Electric motors and generators, where winding inductance governs torque dynamics and transient response
• Wireless power transfer and inductive charging for consumer electronics and electric vehicles