RESISTIVITY

electrical resistance of a conductor of unit cross-sectional area and unit length. A characteristic property of each material, resistivity is useful in comparing various materials on the basis of their ability to conduct electric currents. High resistivity designates poor conductors. Resistivity, commonly symbolized by the Greek letter rho, r, is quantitatively equal to the resistance R of a specimen such as a wire, multiplied by its cross-sectional area A, and divided by its length l; r = RA/l. The unit of resistance is the ohm. In the metre-kilogram-second (mks) system, the ratio of area in square metres to length in metres simplifies to just metres. Thus, in the metre-kilogram-second system, the unit of resistivity is ohm-metre. If lengths are measured in centimetres, resistivity may be expressed in units of ohm-centimetre. The resistivity of an exceedingly good electrical conductor, such as hard-drawn copper, at 20 C (68 F) is 1.77 10-8 ohm-metre, or 1.77 10-6 ohm-centimetre. At the other extreme, electrical insulators have resistivities in the range 1012 to 1020 ohm-metres. The value of resistivity depends also on the temperature of the material; tabulations of resistivities usually list values at 20 C. Resistivity of metallic conductors generally increases with a rise in temperature; but resistivity of semiconductors, such as carbon and silicon, generally decreases with temperature rise. Conductivity is the reciprocal of resistivity, and it, too, characterizes materials on the basis of how well electric current flows in them. The metre-kilogram-second unit of conductivity is mho per metre, or ampere per volt-metre. Good electrical conductors have high conductivities and low resistivities. Good insulators, or dielectrics, have high resistivities and low conductivities. Semiconductors have intermediate values of both.