英文科学读本 第五册·Lesson 07 Conductors and Non-Conductors

Lesson 07 Conductors and Non-Conductors

I want you to think about that poker in the fire, and what it taught us, said Mr. Wilson.

The heat travelled from one end of the poker to the other, sir, said Will. "One particle became heated, and then passed on the heat to the next, and so on. This is the manner in which heat travels through all solid bodies. We call it conduction."

Yes, sir, said Fred, "but heat does not travel equally well through all solids. Some are good conductors, some bad conductors, and some do not conduct or carry away heat at all. We call them non-conductors."

I am glad you have not forgotten your lesson, boys, said Mr. Wilson. "We will now have another little experiment. I have here a basin of ice, and in it are some pieces of metal, marble, stone, brick, wood, cork, leather, and wool. You shall take them out and hold them in your hands, one by one. I want you to remember that, as they all come out of the ice, they must all be at the same temperature—the temperature of the ice. If we place the thermometer in the ice, and against each of them in succession, it will in every case register 32°F. Now, as you remove them from the basin, you shall tell me what you observe."

The piece of metal feels intensely cold, sir; the marble and stone don't seem to be nearly so cold as the metal.

Quite right, said Mr. Wilson. "Now try the other things."

The brick and the wood are not so cold to handle as the stone was, said Fred, "and the leather and wool feel almost warm in comparison."

Right again, boys, said Mr. Wilson. "Now let us see what all this means. In the first place we must remember that every one of these bodies is colder than the hand, and they all rob the hand of some of its heat. The metal, however, is the best conductor, and takes away the greatest amount of heat from the hand, the other substances taking less and less in proportion to their lower conducting power.

The great thing to remember is that the hand feels the sensation of cold in proportion to the amount of heat abstracted from it. We will next place the same things in a basin of hot water from the kettle, and proceed in a similar way.

The metal now feels hotter than the other substances; the leather, cork, and wool convey the least heat. The explanation in this case as in that above clearly depends upon the difference in the conducting powers of the different bodies. It was the knowledge of the superior conducting power of metals that enabled Sir Humphry Davy to construct his safety lamp. You remember, of course, that, in our early lessons on the coal-mine, we first learned to know the use of the Davy lamp, and in our later lessons on coal-gas I took you further, and showed you something of the construction of the lamp itself in relation to the fire-damp in the mine. Our lesson on the conduction of heat will now help you to grasp fully the whole principle of the lamp.

The flame is enclosed in a covering of gauze made of metal wire, and the lamp owes its usefulness entirely to the high conducting power of this metal wire. The explosive gases enter into the lamp through the gauze, and burn there, but the flame, instead of passing outside, is dissipated or conducted away by the wire. Stone, as we have seen, is a better conductor than brick or wood. Hence a stone house (unless the walls are very thick) is colder in winter and hotter in summer than one built of brick or wood.

Ice-houses (or pits) for storing ice are built of brick and thatched with straw, because the low conducting power of brick and straw prevents the warmer air around from acting on the ice. In the hot summer weather, too, the ice is wrapped in blankets and covered with sawdust. Fire-bricks are much used for lining the backs of stoves, because they are very bad conductors, and they prevent the heat from escaping backwards into the chimney.

Metal tea-pots, coffee-pots, kettles, and other vessels for cooking and boiling purposes usually have their handles made of bone, horn, wood, or some other non-conducting substance, and, if not, we are always careful to make a thick pad of flannel, cloth, or paper, to serve the purpose. In the same way, all tools which have to be made hot, such as soldering and branding irons, are provided with wooden handles.

We keep our tea hot on the table without any fire, by merely covering the teapot with a cosey made of wool or down. Both of these substances have very low conducting powers—indeed they are classed as non-conductors. This will explain why in winter we clothe ourselves in fur cloaks, flannel, and other woollen garments. These things are not warm in themselves; they merely act as non-conductors, and prevent the heat from passing away out of our bodies.

In the heat of summer, when we wish to be cool, we exchange our furs and flannels for light, thinner materials, of higher conducting power, and these allow some of the heat of the body to escape into the air.