“Yes, father,” said Rollo, “I understand what radiation is; now tell me about conduction.”
“Very well,” said his father; “suppose you had an iron crowbar here, and should put one end into the fire, among the burning coals, and let it remain there while you took hold of the other end with your hand. Now, the heat would gradually extend along the bar, from the hot end in the fire towards your hand, through the substance of the iron.”
“Would it, sir?” said Rollo.
“Yes,” replied his father; “so that at last the end in your hand would begin to feel hot. Did you never try it with a pin in the lamp?”
“Yes, sir,” said Rollo; “if I hold a pin in the lamp, it burns my fingers.”
“The heat,” said his father, “is conducted along through the pin to your fingers. There is a radiation from the lamp at the same time, but the radiation is not enough to burn you at that distance. That is, if you hold your finger near the lamp, as near as the length of a pin, you would feel the warmth produced by the radiation of the heat through the air, but it would not be enough to burn you. But when you take a pin, and put one end into the flame, it conducts off a great deal more heat than will come by radiation, and your fingers are burned. But then it takes more time for the heat to come along the pin, than for it to come through the air. Thus, in that case, the heat that is radiated comes very quick, though there is but little that comes; while that which is conducted comes slowly, but in a great quantity.”
“It burns me pretty quick, father,” said Rollo.
“Yes,” replied his father, “because the pin is very short. But if it was a knitting-needle, or an iron rod longer still, it would take some time for the heat to pass from one end to the other. Heat moves quickly by radiation, and slowly by conduction.
“Now, if you should go to the stove,” continued Mr. Holiday, “and put your hand upon it, you would burn it by conduction; that is, the heat would be conducted through the iron to your hand. But if you should hold your hand before the fire so near that the heat would come from the coals through the air to it, burning you, then you burn your hand by radiation.
“But you must understand,” continued his father, “that there may be conduction from air, as well as from iron. If you come into a room which is very warm,—that is, where all the air in it is very warm, although the fire may be gone out,—then the warmth you feel comes to your face and hands by conduction from the air which touches your face and hands, just as your hand would be burned by touching the iron that is hot. But if you come into a room where the air is not very warm, but where there is a great blazing fire, then you feel warmed by radiation. Do you understand this?”