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 is reduced, with this behaviour governed by the Knudsen number, defined as formula_27, where formula_28 is the mean free path of gas molecules and formula_29 is the typical gap size of the space filled by the gas. In a granular material formula_29 corresponds to the characteristic size of the gaseous phase in the pores or intergranular spaces. Isotopically pure diamond can have a significantly higher thermal conductivity, increasing from 2,200 W·m·K (for pure natural diamond, 98.9% C) to 3,320 (for 99.9% enriched), possibly 41,000; with 200,000 "predicted" for 99.999% C.) At the atomic level, there are no simple, correct expressions