electrical resistivity
Electrical resistivity
The electrical resistance offered by a homogeneous unit cube of material to the flow of a direct current of uniform density between opposite faces of the cube. Also called specific resistance, it is an intrinsic, bulk (not thin-film) property of a material. Resistivity is usually determined by calculation from the measurement of electrical resistance of samples having a known length and uniform cross section according to the following equation, where ρ is the resistivity, R the measured resistance, A the cross-sectional area, and l the length. In the mks system (SI), the unit of resistivity is the ohm-meter. Therefore, in the equation below, resistance is expressed in ohms, and the sample dimensions in meters. 
The room-temperature resistivity of pure metals extends from approximately 1.5 × 10-8 ohm-meter for silver, the best conductor, to 135 × 10-8 ohm-meter for manganese, the poorest pure metallic conductor. Most metallic alloys also fall within the same range. Insulators have resistivities within the approximate range of 108 to 1016 ohm-meters. The resistivity of semiconductor materials, such as silicon and germanium, depends not only on the basic material but to a considerable extent on the type and amount of impurities in the base material. Large variations result from small changes in composition, particularly at very low concentrations of impurities. Values typically range from 10-4 to 105 ohm-meters. See Electrical resistance, Semiconductor
The temperature coefficients (changes with temperature) of resistivity of pure metallic conductors are positive. Resistivity increases by about 0.4%/K at room temperature and is nearly proportional to the absolute temperature over wide temperature ranges. As the temperature is decreased toward absolute zero, resistivity decreases to a very low residual value for some metals. The resistivity of other metals abruptly changes to zero at some temperature above absolute zero, and they become superconductors.
Metals, and some semiconductors in particular, exhibit a change in resistivity when placed in a magnetic field. Theoretical relations to explain the observed phenomena have not been well developed.