Electric Properties of Solids
Solids may be classified in terms of their resistivity or conductivity as conductors, insulators, or semiconductors. Closer examination of the microscopic conditions for Ohm's law involves free electron density in solids.
These classifications of solid materials can be visualized in terms of the band theory of solids and are strongly correlated with the interatomic spacing in the solid. The electron energy levels in a solid are often expressed in relation to the Fermi energy. The conducting electron density in a metal can be calculated from the Fermi energy.
A current-carrying conductor placed in a magnetic field will exhibit a transverse voltage from the magnetic force exerted on the moving charges. Called the Hall effect, this voltage is proportional to the magnetic field strength and is used in Hall probes for the measurement of magnetic fields.