Electrical resistivity and conductivity conductivity in a solution. A rough summary is as follows: This table shows the resistivity (ρ), conductivity and temperature coefficient of various materials at 20 °C (68 °F, 293 K) The effective temperature coefficient varies with temperature and purity level of the material. The 20 °C value is only an approximation when used at other temperatures. For example, the coefficient becomes lower at higher temperatures for copper, and the value 0.00427 is commonly specified at . The extremely low resistivity (high conductivity) of silver is characteristic of metals. George Gamow tidily summed up the nature of the metals' dealings with
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