Electrical conductivity is a material property that quantifies how readily electric charge moves through a substance when an electric field is applied. In continuum form it appears in Ohm's law for materials as J = σE, where J is the current density, E is the electric field and σ is the conductivity. The SI unit of conductivity is the siemens per metre (S·m⁻¹), named after Werner von Siemens. Conductivity is the reciprocal of resistivity, and its value is intrinsic to the material though the measured value may depend on temperature, frequency and microstructure.

Symbols and basic relations

Common symbols for electrical conductivity are σ, κ and γ. For a simple uniform conductor of length L and cross-sectional area A, the conductance G between its ends is G = σA/L. Conductivity relates current density to electric field, while conductance is a circuit quantity measured between terminals. The reciprocal quantity, resistivity ρ, satisfies ρ = 1/σ and is often tabulated for engineering materials.

Microscopic origin and categories of materials

Conduction requires mobile charge carriers. In metals, conduction electrons in partially filled energy bands respond to applied fields and dominate σ. In electrolytes and molten salts, ions move under the field and determine conductivity; concentration and mobility of ions are key factors. Semiconductors have carrier densities that can be strongly modified by doping, temperature, or illumination, so their conductivity is highly tunable. Insulators lack sufficient free carriers at ordinary conditions and exhibit very low conductivity.

Temperature and structural effects

Temperature typically alters conductivity: in ordinary metals increasing temperature increases lattice vibrations and electron scattering, reducing σ; in intrinsic semiconductors higher temperature generates more carriers and thus increases σ. Microstructural features such as impurities, grain boundaries, porosity and phase composition also influence the effective conductivity of real materials. Anisotropic crystals and composites may require a conductivity tensor to describe direction-dependent response.

Frequency dependence and complex conductivity

Under time-varying fields, conductivity can depend on frequency. The complex conductivity relates the phase and amplitude of current to an oscillating electric field and is important for AC transport, dielectric response and impedance spectroscopy. Frequency dependence gives rise to phenomena such as skin effect in conductors and dispersion in electrolytes and dielectrics. Measurements across frequencies reveal carrier dynamics, relaxation processes and electrode effects.

Measurement techniques and practical considerations

Common methods for determining conductivity include two- and four-point probe measurements for solids, impedance spectroscopy for frequency-resolved properties and dedicated conductivity meters for liquids. The four-point probe reduces the influence of contact resistance when measuring low-resistance samples. For electrolytes, cell geometry, electrode polarization and temperature must be controlled; conductivity is often reported together with concentration and composition. When reporting solid-state conductivity, it is important to note sample orientation for anisotropic materials and the measurement temperature.

Applications and significance

Conductivity is central in design and diagnostics across physics, chemistry and engineering. High-conductivity materials are used for electrical wiring, bus bars and connectors. Semiconductors rely on controlled conductivity for electronic devices. Measurements of water and soil conductivity are widely used in environmental monitoring and agriculture to assess salinity and contamination. Corrosion testing and materials characterization frequently use electrical methods. In geophysics, conductivity contrasts help image subsurface structures. Standards and calibration procedures support consistent measurements in laboratories and industry.

Distinctions and notable points

  • Conductivity vs conductance: σ is intrinsic; conductance G depends on geometry and contacts.
  • Resistivity: ρ = 1/σ, commonly tabulated alongside conductivity for engineering reference.
  • Anisotropy and tensors: In many crystals and composites conductivity is a second-rank tensor, varying with direction.
  • Superconductors: Certain materials exhibit zero dc resistivity below a critical temperature, giving effectively infinite DC conductivity for steady currents.

For further reading, standards and reference material see the list below. These links point to general topics, units and measurement methods relevant to electrical conductivity and to related electrical quantities: