RADIATION AND CONDUCTION.
There are two ways by which the heat or the warmth of one body passes
off to the other bodies around it. One is called _radiation_, the other
_conduction_. Rollo learned the meaning of these terms, and the
distinction between them, under the following circumstances:—
One pleasant morning, late in the autumn, Rollo’s father said that he
was going away on some business, about fifteen miles, and he told
Rollo’s mother that, if she would like to take a ride, he should be very
glad to have her go too. She said that she should like to go very much,
and she proposed that Rollo should go with them. Mr. Holiday had no
objection to this arrangement, and accordingly Jonas was ordered to put
both horses into the carryall, and bring it up to the front door.
“And, on the whole, Jonas,” said Mr. Holiday, “you may go with us too,
and that will relieve me of all care of the horses.”
There were two seats to the carryall. Rollo sat with Jonas on the front
seat, and his father and mother behind. They had a very pleasant ride in
the forenoon, but at noon it looked rather cloudy, as if it might be
going to rain. Rollo said he was afraid that it would rain before they
should get home.
“No matter for that,” said Jonas.
“Why, then we shall get wet,” said Rollo.
“No matter for that,” said Jonas.
They were detained longer than they had expected to be at the place
where Mr. Holiday had his business to do, so that it was rather late
when they set out on their return home. After they had rode about an
hour, it looked still more like rain, and at length, Rollo felt a little
drop upon his cheek. He said, in a complaining tone,—
“There, Jonas, it _is_ going to rain, for it begins to sprinkle now.”
“Rollo,” said Jonas, “when it rains, the best way is to let it rain; for
there is no knowing what good may come of it.”
“I don’t think any good can come to us,” said Rollo, “to be out in this
storm, and night coming on. I should think that you might drive faster.”
Jonas was driving pretty fast, and the rattling of the wheels made so
much noise, that Rollo’s father and mother did not hear what he and
Jonas were saying. Nor did Rollo hear the conversation in which his
father and mother were engaged. They were, in fact, discussing the
expediency of not attempting to get home that evening, but of stopping,
instead, at a little village a few miles before them. As it was growing
cold and late, and as there was every appearance of an approaching
storm, they concluded to do this; and so, to Rollo’s surprise, when they
came into the village, his father said to Jonas,—
“Jonas, you may drive up to this hotel on the right; we are going to
stop here for the night.”
Jonas and Rollo were both glad to hear this. Jonas drove up to the door,
and Rollo and his father and mother were soon transferred from their
places in the carryall, which were beginning to be cold, and cheerless,
and uncomfortable, to a pleasant little parlor, with a bright fire in
the fireplace, and a table in the middle of the room spread for tea.
Now, it happened that, in this little parlor, the fire was made in what
might be called a fireplace-stove, that is, a stove made in the form of
a fireplace. It stood out upon the hearth, and from the top of it a
stove pipe ascended towards the top of the room, and there passed back
into the chimney.
The fire was not built in this room until the party which was to occupy
it arrived, but the room became warm very quick. Rollo was surprised
that the room became warm so quick, and his father said that such an
arrangement as that, namely, a stove standing out upon the hearth, with
a pipe above, warmed a room very quickly; and one reason was, that it
operated by conduction as well as by radiation, whereas a fire in a
common fireplace could only warm the room by radiation. Then Rollo
wanted to know what _conduction_ and _radiation_ meant. His father
explained them to him while they were waiting for the tea to be brought
in.
“There are two ways by which heat moves,” said his father. “One way is
by shooting swiftly through the air like light. Hold your hand up before
the fire.”
So Rollo held his hand up before the fire as his father had directed.
“Do you feel the warmth of the fire upon your hand?”
“Yes, sir,” said Rollo.
“The heat comes from the fire to your hand very swiftly through the air,
and makes your hand feel warm.”
“How do you know it comes so swift, sir?” said Rollo.
“I will show you,” said his father.
So his father looked about the room, and he saw a large book lying upon
a table between the windows. He got the book, and brought it to the
fire.
“Now,” said he, “I will hold the book between your hand and the fire.
Then the heat cannot come to your hand.”
So Mr. Holiday held the book between Rollo’s hand and the fire, and
Rollo said that he could not feel the heat.
“Now,” said his father, “I will take the book away suddenly, and we will
observe what will be the effect. If the heat moves slowly through the
air, it will be some time before you begin to feel the warmth; but if it
shoots through to your hand quick, you will feel it immediately.”
So saying, Mr. Holiday suddenly withdrew the book.
“Yes, sir,” said Rollo; “I feel it immediately.”
Rollo’s father then told him to go back from the fire as far as he could
go, and still feel the heat upon his hand, and then he interposed the
book between the fire and his hand just as before. Rollo still found
that he could feel the heat from the fire instantly, after his father
took the book away.
“Now, that is radiation,” said Mr. Holiday.
“Radiation?” replied Rollo.
“Yes,” said his father. “Heat radiates from a fire, in all directions,
through the air. Heat radiates, too, from all bodies that are hot, or
warmer than the air around them. Go and hold your hand near the stove
pipe, and see if you can feel the heat radiating from the stove.”
Rollo did so. He said he could feel the heat very plainly, though he had
to hold his hand nearer than was necessary when he was feeling the heat
of the fire.
“Yes,” said his father; “that is because the pipe is not so hot as the
fire. If the pipe was as hot as the fire, it would be red hot, and then
you could feel the radiation as far as you do that of the fire. However,
there is some radiation, though the pipe is not very hot. There is
always radiation when the thing is warmer than the air around it. There
is radiation from your face.”
Here Rollo put his hand up towards his face, to try whether he could
feel the radiation.
“You can’t feel it very well with your hand,” said his father, “because
your hand is warm, and so that radiates too. But if you hold your hand
up very near to your face, perhaps you will feel both to be warmer, for
your hand will radiate to your face, and your face will radiate to your
hand.”
Rollo then put his hand up very near to his cheek, and, after holding it
there a minute, he said that he perceived that it felt warm.
“At any rate,” said his father, “you understand what radiation is. The
heat that comes from the sun is radiated.”
“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?”
“Yes, sir,” said Rollo. “I believe I do.”
“I will ask you some questions then, and see if you can answer them.
When we toast bread, is the heat applied by radiation or conduction?”
“By——radiation, sir,” said Rollo.
“Yes; when we bake bread in an oven, is it by radiation or conduction?”
“By conduction,” said Rollo.
“Yes,” said his father, “because you perceive that the fire has all been
taken out from the oven, and the bread is baked by the heat of the air
which surrounds it. I don’t know but that there may be also some
radiation from the sides of the oven, which are very hot; but no doubt a
great deal of the heat is conveyed to the bread by conduction.
“When things are boiled, is the heat applied by radiation or
conduction?” said his father.
“By conduction from the water,” said Rollo.
“Yes; and when they are roasted?”
“By radiation,” replied Rollo.
“Very well,” rejoined his father. “I believe you understand the
distinction very well. Some other time, I will tell you more about both
radiation and conduction. But now I see the tea is coming in.”
Rollo looked around, and saw a girl coming in, bringing a waiter with
two tea-pots upon it, and a plate of toast covered with a bright cover,
and some other things. She put them all upon the table, and then said to
Mr. Holiday,—
“Your tea is ready, sir.”
So Rollo and his father and mother took their places at the tea-table to
eat their supper. Nothing more was said on the subject, until, just
before they rose from the table, Rollo happened to think that his father
had said, at first, that a stove like the one in that room warmed the
room both by radiation and conduction, and he asked his father how that
was.
“In the first place,” replied his father, “it warms the room by
radiation just like any other fire, as you observed by holding your hand
before it. But then it also warms the air in the room by conduction, in
this way; the heat of the fire heats the iron of the stove and of the
pipe. The heat is conducted through the iron to the outside, and there
some of it radiates, and the rest is conducted into the air which
happens to touch it. Exactly as if you put your hand against the stove
pipe, the heat is conducted through to your hand. So it is conducted to
the air, which touches the pipe. This air becomes heated, and swells,
and so grows light, and rises out of the way, and more cold air comes in
to the stove pipe, and gets heated in the same way by conduction. So the
air is continually coming up against the stove and pipe at all parts,
and it gets heated by conduction, and then rises and floats away over
the room, and thus helps warm the room. But in a common fireplace all
the heat which is conducted off goes back among the bricks of the
chimney, and does not help warm the room. If we had a light tuft of down
or cotton, and were to put it near the pipe, perhaps it would be carried
up to the top of the room, by the current of air which is heated by
conduction of the pipe.”
“I wish I had a tuft to try it,” said Rollo. “It would be a very good
experiment.”
Rollo had no tuft, but he had that evening an opportunity to witness the
effect of the ascending current of heated air, in a way which he did not
anticipate; for, after tea, they had the table out, for Rollo’s father
to do some writing, while his mother sat in a rocking-chair near,
reading. Rollo wanted to write, and so his father let him have a pen and
ink. He said that he would write a little note to his cousin Lucy. It
was a very small note, on a little piece of paper, and when he had
written down one side, he undertook to dry what he had written by means
of the radiation from the lamp, and accordingly held up his paper very
near the flame of the lamp, upon one side. The writing, however, did not
seem to dry very fast, and in changing the position of it, he held it
over the flame, though so high that he thought that there was no danger
of burning it. But the current of heated air which rises upward from a
fire, even so small a fire as the flame of a lamp, is hotter, and rises
higher, than most children would suppose. At any rate, before Rollo was
aware of the danger, his note was on fire, and he was obliged to turn
around suddenly, and throw his paper upon the hearth, to escape burning
his fingers.
QUESTIONS.
In what two ways is heat conveyed from one body to another? Relate
the circumstances which led Rollo and his father and mother to spend
the night at a tavern. What sort of a stove did they find in the
little parlor? What is radiation of heat? How did Mr. Holiday
demonstrate it to Rollo? How did he prove that heat passes swiftly
when it is radiated? How did Mr. Holiday explain and illustrate
conduction? By which mode is the greatest quantity of heat conveyed?
Which communicates heat most rapidly? What were some of the
questions which Mr. Holiday put to Rollo to ascertain whether he
understood the distinction? What incident occurred to Rollo in the
evening?