# What are the worst heat conductors

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### Conduction

[380]Conduction, the propagation of heat in the body by giving off particle to particle. Of all bodies, metals are the best conductors of heat; Wood, ashes, straw, silk, feathers, hair, wool, etc., generally the loose bodies from the animal and plant kingdom, are the worst conductors of heat; Stones, glass, porcelain conduct a little better. If a metal rod is heated at one end, and the temperature of the same is determined at different points by means of thermometers (t, t1, t2 etc., see fig.), which are inserted into the drill holes of the rodA B are sunk, after some time every thermometer reaches a fixed position, and a state of equilibrium occurs in the heat distribution along the rod, which is due to the fact that every cross-section of the rod from the heat source flows in as much heat as it does after the on the other side and continuously loses it through its surface to the colder surroundings. A distinction is made between an internal and an external thermal conductivity, and by internal conductivity the amount of heat that passes through a cube of the substance from 1 cm side in the unit of time (second, minute) when two opposite surfaces have a temperature difference of 1 ° and the others Surfaces are thought of as impermeable to heat; External conductivity, however, means the amount of heat that a body gives off in one minute with a temperature excess of 1 ° over its surroundings through 1 square centimeter of its surface.

Dissemination of heat by conduction.

If the above metal rod has reached thermal equilibrium or the "steady state", it follows that if the distances from the heat source (L) increase in arithmetic series, the corresponding temperature increases decrease in geometric series, a law that is represented by the curved line a, a1, a2 etc., which connects the peaks of the mercury columns of the thermometers, is visualized. For rods of different metals with the same dimensions and the same surface properties, the thermal conductivity behaves like the squares of the distances from the heat source, at which the same temperature surpluses are observed under otherwise identical circumstances. The amount of heat, measured in small (gram) calories, that passes per square centimeter and second through a flat plate 1 cm thick, the two sides of which have a temperature different by 1 °, is:

The different thermal conductivity of the body is often put to practical use. Metal teapots, oven doors and poker have wooden handles; Trees and bushes are wrapped in straw in winter to protect them from freezing. [380] On the other hand, poor heat conductors prevent external heat from penetrating the ice cellar. Fireproof safes contain ash between their double walls, which delays the entry of heat. In a cold room the metal doorknob feels colder than the table carpet, although both are at the same temperature, because the metal conducts the heat from our hands more quickly and therefore draws more heat from the hand than the poorly conductive fabric. If one holds one of his wire mesh in a gas flame, it appears as if it was cut off; the metal threads dissipate the heat so quickly that the flame gases are cooled below their ignition temperature. If the gas is allowed to flow out of the burner without lighting it and the wire net is held in the gas stream, the latter can be ignited above the net without the ignition propagating under the net. The Davy safety lamp is based on this behavior (see d.). Liquids are poor conductors of heat; in them the heat spreads preferentially through currents (convection), which arise from the fact that when heated from below, the liquid particles, which have become specifically lighter through expansion, rise upwards and are replaced by the colder particles that sink; The heating of a liquid is immensely promoted by this cycle. If, on the other hand, it is heated from above, the heat only spreads very slowly downwards due to the poor conductivity (see water, p. 402). Gases also conduct heat very poorly; dormant layers of air, such as B. the layer of air enclosed between double windows, are very suitable to prevent the dissipation of heat. Our clothes owe their "warmth" property primarily to the poorly conducting air that is held in between them. Steam pipes are secured against strong heat loss by using asbestos, slag wool, silk waste and similar thermal insulation compounds. The vacuum offers excellent thermal protection. This is the basis for the Dewarschen bottles for storing liquid air, as well as for thermos bottles for keeping milk warm, keeping ice cream cold, etc.