英文科學讀本 第五冊·Lesson 03 Conduction of Heat

Lesson 03 Conduction of Heat

We have already learned something about heat and its effects on various bodies—solids, liquids, and gases, said Mr. Wilson. "Our next business will be to inquire how heated bodies part with their heat—that is, how heat passes from one portion of matter to another. I want you to come and take the poker out of the fire, Fred. That's right. But why did you drop it? Hot, was it? How can that be? The part that was in the fire is of course red-hot, but you held the handle, and that was out of the fire. Let us see what it means. Hold one end of this copper wire in your hand, keeping the other end in the flame of the Bunsen burner. Tell us what happens."

The end that I hold in my hand is getting hot, sir.

Yes, Fred, it is, and it will soon be so hot that you will be glad to drop it. The fact is, in both cases the heat from the fire and the burner travels along the metal, from particle to particle, one particle first getting hot itself and then heating the next, and so on, until it reaches the opposite end. I will now show you a very interesting little experiment. I have here a long strip of copper and another of iron joined, end to end, by means of a rivet. I will fix them so that the joined part is in the flame of the Bunsen burner, and on the opposite ends I will place a small piece of phosphorus. In a short time the phosphorus on the copper takes fire, but that on the iron will remain much longer before it ignites.

Now, what do we learn from this? We learn that heat travels along the particles of both the copper and the iron, but that it travels more rapidly along the former than the latter. Let us go a little farther. I will place this tin dish on the stand over the Bunsen burner, and put a piece of bone and a thin strip of copper into the dish. Both are receiving the same amount of heat, and it is only natural to expect that one will be exactly as hot as the other. Now, Fred, take them out of the dish, the bone first, and then the copper. Tell us what you observe."

The bone is not heated at all, sir, except in the very spot where it rested on the dish.

Quite right, Fred. You can easily pick up the bone. Now take the piece of copper out. Ah! you drop that quickly enough, don't you? The copper, you see, is not heated in one spot only, but the whole of it is hot. The heat travels rapidly along the particles of copper, but very slowly indeed through the bone. Copper and iron carry, or conduct, the heat from particle to particle of their substance. We say that these metals are good conductors of heat, and that heat travels along them by conduction. The bone conducts heat so slowly that we usually call it a non-conductor. There are some bodies which, although not actually non-conductors, have such small conducting power that we arrange them in a class by themselves as bad conductors.

In classifying bodies according to their conducting power, we find that dense substances are generally the best conductors; light, porous ones the worst. Conduction may be said to be confined to solid bodies, liquids and gases being very bad conductors of heat.

The metals are the best conductors of all, but they differ very much one from the other. They stand thus in the order of their conducting power: silver, copper, gold, brass, tin, iron, lead, platinum, and bismuth—silver being the best. Among the bad conductors are marble, stone, brick, glass, earthenware, sealing-wax, leather, wood, linen, cotton, and straw. The non-conductors include bone, horn, feathers, down, fur, wool, flannel, silk, hair, cork, india-rubber, and air."