CHAPTER X

TRANSMISSION OF HEAT-CONDUCTION

68. Modes of Transmission of Heat.-Heat may be transmitted from one point to another by three processes, called respectively conduction, convection, and radiation.

You know that if the point of a poker be pushed into a fire, the heating effect is not confined to the point alone: it gradually extends throughout the length of the poker, from the parts near the point towards the handle, which, after a while, may become too hot for you to hold. This transference of heat from hotter to colder parts of the poker, and from the hot poker to your hand, is called conduction.

Again, if you withdraw the red-hot poker from the fire and hold your hand above it, you feel a sensation of heat which is mainly due to the warm air (heated by contact with the poker) rising upwards. This process is called convection.

Lastly, if you hold your hand an inch or so below the redhot poker you still feel a sensation of heat. This is not due to convection (for heated air always rises upwards), but is produced by a direct transmission of heat through the air from the poker to your hand, a process which is called radiation. It is by radiation that your hands are warmed when you hold them in front of the fire.

69. Definition.— Conduction is the transmission of heat from hotter to colder parts of a body, or from a hot body to a colder body in contact with it. This transmission takes place gradually from particle to particle, but without any visible motion of the parts of the body.

The last sentence is added to point out how conduction differs from radiation and convection. Conduction is a gradual process in which heat only passes from hot to cold parts of the body by heating the intermediate parts. The transmission of heat by radiation is so rapid as to be practically instantaneous; it also takes place without heating the medium through which it passes. In the case of convection heat is conveyed from one point to another by actual motion of the hot body as a whole. These distinctions will be better understood after reading the next two chapters.

70. Conduction in Solids.-Solids differ from one another enormously in their power of conducting heat. Metals are generally good conductors, silver and copper being the best. Glass, stone, leather, wood, flannel, and organic substances generally are bad conductors.

EXPT. 41. Hold one end of a copper or brass wire, about 3 inches long, in a flame. The heat is rapidly conducted along the wire, and it soon becomes too hot to hold. An iron or platinum wire does not get hot so quickly. A strip of wood (a match) can be held until it burns down quite near the fingers. A piece of glass rod or tubing can be held quite comfortably for a long time. It is this bad conducting power of glass that makes glass-blowing possible. When a glass tube is fused in a blowpipe-flame, it can be handled to within an inch or so of the fixed part without discomfort.

EXPT. 42.--Place spoons made of different substances with their bowls dipping into hot water. The handle of a silver spoon (real silver) soon gets so hot that you cannot comfortably hold it. A common spoon (made of Britannia metal) does not get hot so soon; with spoons made of bone or wood the heating effect is scarcely noticeable.

EXPT. 43.-Paper is a bad conductor of heat, and at once becomes scorched and begins to burn when placed in a flame. Yet a piece of paper may be held for some time in a flame without burning (and even without getting scorched) if there is a metal surface immediately behind it to carry away the heat.

This can be well shown by means of a cylinder one half of which is made of brass and the other half of wood (or one end

of a wooden cylinder may be turned down so that a brass tube of the same diameter can be slipped over it). Wrap a piece of writing-paper tightly round the cylinder and hold it in a Bunsen flame, as in Fig. 40. The paper covering the wooden half of the cylinder is scorched (just up to

the line of junction) long before any effect is produced on the other half. The brass conducts away the heat so rapidly that the paper is kept cool. The experiment shows that brass is a far better conductor of heat than wood is.

71. Comparison of Conducting Powers.--The following experiments show how the conducting powers (or conductivities) of different materials may be compared.

Fig. 40.

EXPT. 44.-A number of metal and other rods, of the same length and thickness, are covered with a coating of wax, and are introduced into holes in the front of a metal trough (Fig. 41), which is then filled with boiling water. As the heat travels along each rod and warms it up to the proper temperature, the wax melts.

Wait until there is no further sign of melting, and then observe that the wax has melted much farther along some rods than others.

G

EXPT. 45.-Take two strips or bars of iron and copper respectively, and of the same size.

Attach small wooden balls

Fix them end to end

(or marbles) to them by means of wax.

Fig. 42.

and place a burner underneath, so as to heat them equally (a better plan is to rivet both to a cross-piece and heat this).

The balls will be found to drop off the copper strip more rapidly than off the iron.

EXPT. 46.-Cut discs of copper, iron, wood, and cork of about inch thickness, and large enough to cover the top of the air-thermoscope (p. 5). As a source of heat use a metal cylinder (iron or copper) heated to 100° by immersion in boiling

water.

Place one of the discs on top of the air-thermoscope, and on this put the hot cylinder. Wait a couple of minutes and observe the greatest depression produced. Try the other discs in the same way. Copper gives the greatest depression; wood and cork scarcely allow any heat to pass.

72. When one end of a bar is heated, the rate at which its temperature rises depends not only upon its conductivity but also upon its specific heat. Other things being equal, a bar having a low specific heat would get hot more rapidly than another having a high specific heat. Thus the above methods of experimenting are not quite satisfactory, excepting where the substances compared have nearly the same specific heat (as in the case of iron and copper, Expt. 45). But they show sufficiently well that metals are, in general, good conductors of heat, silver and copper being the best; whereas stone, glass, wood, cork, etc., do not conduct heat nearly so well, or are bad conductors.

In order to keep our bodies warm we make use of such bad conductors (woollen clothes, flannel blankets, fur, leather, etc.) Other applications of non-conductors will readily occur to you -e.g. kettle-holders, the handles of coffee-pots, kettles, and soldering-irons, etc. The badly-conducting materials which we use for 'keeping out cold' are also the best for keeping out heat. Thus, if you wish to keep a block of ice in hot weather, and have no ice-chest to put it in, it should be wrapped up in flannel.

You will now be better able to understand why it is that, in our climate, metals and other good conductors feel cold to the touch (see pp. 1-3). If the temperature of surrounding objects were higher than that of our bodies, the reverse would be the case; and so in the hot rooms of Turkish baths it is found that iron and stone are painful to touch because they part with heat so readily; whereas wood and flannel can be handled without discomfort.

73. Action of Wire-gauze on Flame.-A combustible substance will not burn, even in presence of air or oxygen, unless it is raised to a certain temperature, which is called the 'temperature of ignition' of the particular substance. Now when a good conductor is introduced into a flame it rapidly withdraws heat from the flame and thus cools it. The following experiments show that it is easy to cool and extinguish a flame in this way.

EXPT. 47.-Coil some copper wire round a rod, leaving a piece free at the end to serve as a handle, and making the coil of such size as to fit round the flame of a spiritlamp. Lower the coil vertically over the flame until the bottom of it touches the wick. The flame shrinks away from the coil and then goes out.

The coil does not crush out the flame; there is plenty of room for it to burn and air can easily pass between the turns of wire. That the effect is really due to the cooling action of the metal may be shown as follows. Heat the coil to redness by means of a Bunsen burner and lower it again over the flame of the spirit-lamp; it is no longer extinguished. Cool the coil by dipping it in water; dry it, and repeat the experiment. The cold coil at once puts out the flame.