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 reciprocal of thermal conductivity is called thermal resistivity. The general equation for thermal conductivity is formula_1, where formula_2 is the heat flux, formula_3 is the thermal conductivity and formula_4 is the temperature gradient. In the International System of Units (SI), thermal conductivity is measured in watts per meter-kelvin (W/(m⋅K)). Some papers report in watts per centimeter-kelvin (W/(cm⋅K)). In imperial units, thermal conductivity is measured in BTU/(hr⋅ft⋅°F). The dimension of thermal conductivity is MLTΘ, expressed in terms of the dimensions mass (M), length (L), time (T), and temperature (Θ). Other units which are closely related to the thermal conductivity are in