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
Thermal conductivity given thickness of a material, that particular sample's "thermal resistance" and the reciprocal property, "thermal conductance", can be calculated. Unfortunately, there are differing definitions for these terms. Thermal conductivity, "k", often depends on temperature. Therefore, the definitions listed below make sense when the thermal conductivity is temperature independent. Otherwise a representative mean value has to be considered; for more, see the Equations section below. For general scientific use, "thermal conductance" is the quantity of heat that passes in unit time through a plate of "particular area and thickness" when its opposite faces differ in temperature by one kelvin. For a