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 and it is measured in watts per square metre of surface area for a temperature gradient of 1 K per unit thickness of 1 m". Therefore, specific thermal conductivity is calculated as: where: formula_14 = temperature difference (K, or °C) = 1 K during measurement Absolute thermal conductivity, on the other hand, has units of formula_15 or formula_16, and can be expressed as where formula_18 = absolute thermal conductivity (W/K, or W/°C). Substituting formula_18 for formula_20 into the first equation yields the equation which converts from absolute thermal conductivity to specific thermal conductivity: Solving for formula_18, we get the equation