For example, in a homogeneous material, independently of thickness, the thermal conductivity is the rate of heat transfer, in well defined conditions across a unit surface per unit variation of temperature, in a direction perpendicular to the surface.
The conductivity of a good conductor such as silver or copper may be more than a million times higher than that of an insulator such as glass or mica. The phenomenon of superconductivity is observed when certain substances are cooled to a temperature close to absolute zero: their conductivity then become quasi-infinite.
Pure copper is an excellent conductor of electricity with a conductivity 95% that of silver.
Copper also has a high thermal conductivity. Copper is mainly used for its electrical conductivity, the highest among industrial metals. Thus it is used in such electrical applications as cables, wires and electrical devices.
Because of its high thermal conductivity, aluminium is used for cooking utensils and the pistons of internal combustion engines. For a given size of wire, the conductance of aluminium is 63% that of copper, but its mass is 50% lower. For a given conductance, an aluminium wire will be thicker than a copper wire, but will still be lighter. This lightness is of special interest in the transmission of high voltage electric power over long distances. Aluminium conductors are now used to transmit electricity at 70 000 volts or more.