Electrical resistivity and conductivity ions. In biological membranes, currents are carried by ionic salts. Small holes in cell membranes, called ion channels, are selective to specific ions and determine the membrane resistance. The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing in a loop of superconducting wire
Electrical resistivity and conductivity inside the conductor. Conductivity is the inverse (reciprocal) of resistivity. Here, it is given by: For example, rubber is a material with large "ρ" and small "σ"—because even a very large electric field in rubber makes almost no current flow through it. On the other hand, copper is a material with small "ρ" and large "σ"—because even a small electric field pulls a lot of current through it. As shown below, this expression simplifies to a single number when the electric field and current density are constant in the material. When the resistivity of a material has a directional component,