Heat conductivity is basically the ability of a substance, in this case metal, to allow heat pass to pass through it. Different materials conduct heat differently, and each type of metal has a particular conductivity. The different characteristics of specific metals or alloys means their potential industrial uses are extremely varied, which is why we use them in items as different as light bulbs, saucepans and electrical wiring.
Thermal conductivity is measured as the quantity of energy transferred by surface unit and unit time, following a temperature gradient. The units involved are watts (W) per metre (m) per kelvin (k), where kelvin is a type of temperature degree scale. Heat is a form of energy, and some of what carries the energy through the metal down the temperature gradient from hot to cold spots is movement of free excited electrons. The vibration of the molecules in the static structure of the metal pass on vibrational energy to the next molecules down the line, and so on.
Certain pure metals are on top of the conductivity table, with alloys further down the list. Silver is the best metal conductor of heat, with a thermal conductivity of 420 W/mk. Copper is the second-best, with a conductivity of 401 W/mk. After silver and copper, the thermal conductivity capability drops to gold at 317 W/mk, aluminium at 237 W/mk, and tungsten at 174 W/mk. Metal mixtures like bronze and brass are at 110 W/mk and 150 W/mk respectively.
Intermediate and poor conductors
Many familiar metals have conductivities in the 35 to 120 W/mk range. These include iron, tin, lead and carbon steel. Stainless steel is a poor thermal conductor, with a conductivity of about 15 W/mk. This varies depending on the type of stainless steel. Mercury has a very low conductivity of 8 W/mk. In comparison to these seemingly very low conductivity values though, plywood scores a measly 0.11 W/mk, limestone 1.0 W/mk and cotton an almost negligible 0.04 W/mk.
Relative thermal conductivity
People often perceive metals as cold, even though metals conduct heat much better than many other substances, such as wood. This is because of relative thermal conductivity, which means that because metal conducts heat so well, if its initial temperature is cooler than a person's skin, it feels cold to the touch because the heat energy is moving from the skin to the metal. Wood doesn't feel so cold, as it conducts heat from skin less efficiently.