1-5 offered by a cube-shaped conductor 1centimeter in length and 1 square centimeter in cross-sectional area. The unit of volume to be used is given in tables of specific resistances. SPECIFIC RESISTANCE OR RESISTIVITY Specific resistance, or resistivity, is the resistance in ohms offered by a unit volume (the circular-mil-foot or the centimeter cube) of a substance to the flow of electric current. Resistivity is the reciprocal of conductivity. A substance that has a high resistivity will have a low conductivity, and vice versa. Thus, the specific resistance of a substance is the resistance of a unit volume of that substance. Many tables of specific resistance are based on the resistance in ohms of a volume of a substance 1 foot in length and 1 circular mil in cross-sectional area. The temperature at which the resistance measurement is made is also specified. If you know the kind of metal a conductor is made of, you can obtain the specific resistance of the metal from a table. The specific resistances of some common substances are given in table 1-1. Table 1-1.—Specific Resistances of Common Substances Specific resistance at 20ºC. Substance Centimeter cube (microhoms) Circular-mil-foot (ohms) Silver 1.629 9.8 Copper (drawn) 1.724 10.37 Gold 2.44 14.7 Aluminum 2.828 17.02 Carbon (amorphous) 3.8 to 4.1 …………………… Tungsten 5.51 33.2 Brass 7.0 42.1 Steel (soft) 15.9 95.8 Nichrome 109.0 660.0 The resistance of a conductor of a uniform cross section varies directly as the product of the length and the specific resistance of the conductor, and inversely as the cross-sectional area of the conductor. Therefore, you can calculate the resistance of a conductor if you know the length, cross-sectional area, and specific resistance of the substance. Expressed as an equation, the "R" (resistance in ohms) of a conductor is