What Is Resistance?

Resistance is a fundamental electrical property of a material that quantifies its opposition to the flow of electric current. Defined by Georg Ohm in 1827, it is expressed through Ohm's law as the ratio of voltage across a conductor to the current flowing through it: R = V/I. The SI unit of resistance is the ohm (Ω), where one ohm equals one volt per ampere. Resistance is determined by a material's intrinsic resistivity, its geometry (length and cross-sectional area), and its temperature.

Resistance arises from the interaction between conduction electrons and the atomic lattice of a material. In metals, electrons scatter off lattice imperfections and thermally vibrating atoms, converting kinetic energy into heat. Insulators exhibit such high resistance that current flow is negligible under normal voltages, while semiconductors occupy an intermediate range and allow resistance to be controlled through doping or applied fields. This tunability makes resistance a central concept in circuit design, materials science, and sensor engineering.

Electrical Resistance Measurement

Accurately measuring resistance is essential for characterizing materials, calibrating instruments, and manufacturing electronic components. Standard laboratory methods include the two-terminal connection (adequate for resistances above a few ohms) and the four-terminal Kelvin connection (required for low resistances, where lead and contact resistances would otherwise dominate the reading). The Wheatstone bridge, developed in the 1840s, remains widely used for precise ratio measurements and is the basis for most resistance-based sensors. At the highest accuracy levels, the quantum Hall effect provides a primary resistance standard. The NIST Metrology of the Ohm project maintains resistance references traceable to quantum electrical standards and supports more than 250 measurement sites across the United States.

Skin Effect

When alternating current flows through a conductor, the current does not distribute uniformly across the cross-section. Instead, it concentrates near the surface, a phenomenon called the skin effect. This occurs because the changing magnetic field induced by the current creates opposing eddy currents in the conductor's interior, effectively pushing the current toward the outer layer. The depth at which current density falls to 1/e of its surface value is called the skin depth, which decreases with increasing frequency. In copper at 60 Hz, the skin depth is roughly 8.5 mm; at 100 kHz, it drops to about 0.2 mm. IEEE standards such as IEEE Std 80-2013 account for skin effect in grounding conductor design. Practical mitigation strategies include litz wire, which bundles many thin, individually insulated strands to restore the effective cross-sectional area for high-frequency current flow, and published formulations for frequency-dependent resistance in solid cylindrical conductors have been developed through IEEE Xplore research.

Temperature Dependence and Material Resistivity

Resistance in metallic conductors increases approximately linearly with temperature over moderate ranges, described by the relation R(T) = R₀[1 + α(T − T₀)], where α is the temperature coefficient of resistance (TCR). Platinum has a well-characterized, highly stable TCR, which is why platinum resistance thermometers (PRTs) serve as the practical temperature standard for the International Temperature Scale between −200 °C and 660 °C. In semiconductors, resistance typically decreases with rising temperature, because higher temperatures promote more carriers across the bandgap. Superconductors exhibit zero resistance below a critical temperature, enabling lossless current flow. A PMC review of resistance strain force sensor development illustrates how resistivity changes in nickel-chromium and copper-nickel alloys are exploited for precise mechanical measurements.

Applications

Resistance has applications in a wide range of fields, including:

• Temperature sensing, through platinum resistance thermometers and thermistors
• Resistance heating in industrial furnaces, consumer appliances, and food pasteurization
• Non-volatile memory, through resistance change mechanisms in memristors and RRAM devices
• Signal conditioning and voltage division in analog circuit design
• Structural health monitoring via resistance strain gauges on bridges, aircraft, and pressure vessels