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
Conductivity near the percolation threshold critical exponents we have the "correlation length", formula_10 formula_11 and the "percolation probability", P: formula_12 For the description of the electrical percolation, we identify the occupied bonds of the bond-percolation model with the metallic component having a conductivity formula_13. And the dielectric component with conductivity formula_14 corresponds to non-occupied bonds. We consider the two following well-known cases of a "conductor-insulator mixture" and a "superconductor–conductor mixture". In the case of a conductor-insulator mixture we have formula_15. This case describes the behaviour, if the percolation threshold is approached from above: formula_16 for formula_6 Below the percolation threshold we have no conductivity, because