Ohm's Law. The potential difference between any two points in a conductor is directly proportional to the current flowing through it. (This only holds true for an ohmic material. Non-ohmic materials do not display a direct relationship.)

where V is the potential difference measured in volts, I is the current measured in amperes and R is the resistance measured in ohms. Thus, 1 ohm = 1 volt / ampere. The symbol used for the ohm is the Greek letter Omega (W).

Increased resistance in an electric circuit decreases conductance. Resistance and conductance are inversely related. The resistance of a cylindrical resistor depends on its length, its cross-sectional area, its temperature and the resistivity of the material. Resistance is directly proportional to the length of the conductor and varies inversely with its cross-sectional area. Resistance usually increases with temperature. Some materials exhibit superconductivity (i.e., resistance drops to zero) at lower temperatures. (Materials become superconductive at different temperatures.) Resistance depends on the resistivity of the material. The resistivity is defined as the resistance of a cylinder of the material 1 meter long with a cross-sectional area of 1 m². Resistivity varies with temperature. It is measured in Wm²/m.