Michael Faraday. From
The Forces of Matter, Delivered before a Juvenile Auditory at the
Royal Institution of Great Britain during the Christmas Holidays of
1859–60
Vol. 30, pp. 13-21 of
The Harvard Classics
Two sticks, a table,
and a pail were the commonplace implements used by Michael Faraday to
demonstrate great scientific truths.
(Faraday sends
"Experimental Researches" to Royal Society, Nov. 6, 1845.)
Lecture
I.—The Force of Gravitation
I want you now to understand the
nature of the most simple exertion of this power of matter
called weight or gravity. Bodies
are heavy; you saw that in the case of water when I placed it in the
balance. Here I have what we call a weight [an iron
half cwt.]—a thing called a weight because in it the exercise of
that power of pressing downward is especially used for the purposes
of weighing; and I have also one of these little inflated
India-rubber bladders, which are very beautiful although very common
(most beautiful things are common), and I am going to put the weight
upon it, to give you a sort of illustration of the downward pressure
of the iron, and of the power which the air possesses of resisting
that pressure; it may burst, but we must try to avoid that. [During
the last few observations the lecturer had succeeded in placing the
half cwt. in a state of quiescence upon the inflated India-rubber
ball, which consequently assumed a shape very much resembling a flat
cheese with round edges.] There you see a bubble of air bearing half
a hundred-weight, and you must conceive for yourselves what a
wonderful power there must be to pull this weight
downward, to sink it thus in the ball of air.
Let me now give you another
illustration of this power. You know what a pendulum is. I have one
here (FIG. 1), and if I set it swinging, it
will continue to swing to and fro. Now I wonder whether you can tell
me why that body oscillates to and fro—that pendulum bob, as it is
sometimes called. Observe, if I hold the straight stick horizontally,
as high as the position of the ball at the two ends of its journey,
you see that the ball is in a higher position at the two extremities
than it is when in the middle. Starting from one end of the stick,
the ball falls toward the centre, and then rising again to the
opposite end, it constantly tries to fall to the lowest point,
swinging and vibrating most beautifully, and with wonderful
properties in other respects—the time of its vibration, and so
on—but concerning which we will not now trouble ourselves.
Fig. 1
If a gold leaf, or piece of thread, or
any other substance were hung where this ball is, it would swing to
and fro in the same manner, and in the same time too. Do not be
startled at this statement; I repeat, in the same manner and in the
same time, and you will see by-and-by how this is. Now that power
which caused the water to descend in the balance—which made the
iron weight press upon and flatten the bubble of air—which caused
the swinging to and fro of the pendulum, that power is entirely due
to the attraction which there is between the falling body and the
earth. Let us be slow and careful to comprehend this. It is not that
the earth has any particular attraction toward
bodies which fall to it, but, that all these bodies
possess an attraction every one toward the other. It is not that the
earth has any special power which these balls themselves have not;
for just as much power as the earth has to attract these two balls
[dropping two ivory balls], just so much power have they in
proportion to their bulks to draw themselves one to the other; and
the only reason why they fall so quickly to the earth is owing to its
greater size. Now if I were to place these two balls near together, I
should not be able, by the most delicate arrangement of apparatus, to
make you, or myself, sensible that these balls did attract one
another; and yet we know that such is the case, because if, instead
of taking a small ivory ball, we take a mountain, and put a ball like
this near it, we find that, owing to the vast size of the mountain as
compared with the billiard ball, the latter is drawn slightly toward
it, showing clearly that an attraction does exist,
just as it did between the shell-lac which I rubbed and the piece of
paper which was overturned by it.
Now it is not very easy to make these
things quite clear at the outset and I must take care not to leave
anything unexplained as I proceed, and, therefore, I must make you
clearly understand that all bodies are attracted to the earth, or, to
use a more learned term, gravitate. You will not mind my
using this word, for when I say that this penny-piece gravitates, I
mean nothing more nor less than that it falls toward the earth, and,
if not intercepted, it would go on falling, falling, until it arrived
at what we call the centre of gravity of the earth, which I will
explain to you by-and-by.
I want you to
understand that this property of gravitation is never lost; that
every substance possesses it; that there is never any change in the
quantity of it; and, first of all, I will take as illustration a
piece of marble. Now this marble has weight, as you will see if I put
it in these scales; it weighs the balance down, and if I take it off,
the balance goes back again and resumes its equilibrium. I can
decompose this marble and change it in the same manner as I can
change ice into water and water into steam. I can convert a part of
it into its own steam easily, and show you that this
steam from the marble has the property of remaining in the same place
at common temperatures, which water steam has not.
If I add a little liquid to the marble and decompose it ( 6),
I get that which you see—[the lecturer here put several lumps of
marble into a glass jar, and poured water and then acid over them;
the carbonic acid immediately commenced to escape with considerable
effervescence]—the appearance of boiling, which is only the
separation of one part of the marble from another. Now this [marble]
steam, and that [water] steam, and all other steams, gravitate just
like any other substance does; they all are attracted the one toward
the other, and all fall toward the earth, and what I want you to see
is that this steam gravitates. I have here (FIG. 2)
a large vessel placed upon a balance, and the moment I pour this
steam into it you see that the steam gravitates. Just watch the
index, and see whether it tilts over or not. [The lecturer here
poured the carbonic acid out of the glass in which it was being
generated into the vessel suspended on the balance, when the
gravitation of the carbonic acid was at once apparent.] Look how it
is going down. How pretty that is! I poured nothing in but the
invisible steam, or vapor, or gas which came from the marble, but you
see that part of the marble, although it has taken the shape of air,
still gravitates as it did before. Now will it weigh down that bit of
paper? [placing a piece of paper in the opposite scale.] Yes, more
than that; it nearly weighs down this bit of paper [placing another
piece of paper in]. And thus you see that other forms
of matter besides solids and liquids tend to fall to the earth; and,
therefore, you will accept from me the fact that all things
gravitate, whatever may be their form or condition. Now here is
another chemical test which is very readily applied. [Some of the
carbonic acid was poured from one vessel into another, and its
presence in the latter shown by introducing into it a lighted taper,
which was immediately extinguished.] You see from this result also
that it gravitates. All these experiments show you that, tried by the
balance, tried by pouring like water from one vessel to another, this
steam, or vapor, or gas is, like all other things, attracted to the
earth.
Carbonic acid, under
ordinary circumstances, is a colorless invisible gas, about half as
heavy again as air. Dr. Faraday first showed that under great
pressure it could be obtained in a liquid state. Thilorier, a French
chemist, afterward found that it could be solidified.
Fig. 2
There is another point I want in the
next place to draw your attention to. I have here a quantity of shot;
each of these falls separately, and each has its own gravitating
power, as you perceive when I let them fall loosely on a sheet of
paper. If I put them into a bottle, I collect them together as one
mass, and philosophers have discovered that there is a certain point
in the middle of the whole collection of shots that may be considered
as the one point in which all their gravitating
power is centred, and that point they call the centre
of gravity;it is not at all a bad name, and rather a short
one—the centre of gravity. Now suppose I take a sheet of
pasteboard, or any other thing easily dealt with, and run a bradawl
through it at one corner, A (FIG. 3), and
Mr. Anderson holds that up in his hand before us, and I then take a
piece of thread and an ivory ball, and hang that upon the awl, then
the centre of gravity of both the pasteboard and the ball and string
are as near as they can get to the centre of the earth; that is to
say, the whole of the attracting power of the earth is, as it were,
centred in a single point of the cardboard, and this point is exactly
below the point of suspension. All I have to do, therefore, is to
draw a line, A B, corresponding with the string, and we shall find
that the centre of gravity is somewhere in that line. But where? To
find that out, all we have to do is to take another place for the awl
(FIG. 4), hang the plumb-line, and make the
same experiment, and there [at the point C] is the centre of
gravity,—there where the two lines which I have traced cross each
other; and if I take that pasteboard and make a hole with the bradawl
through it at that point, you will see it will be supported in any
position in which it may be placed. Now, knowing that, what do I do
when I try to stand upon one leg? Do you not see that I push myself
over to the left side, and quietly take up the right leg, and thus
bring some central point in my body over this left leg? What is that
point which I throw over? You will know at once that it is the centre
of gravity—that point in me where the whole gravitating force
of my body is centred, and which I thus bring in a line over my foot.
Fig. 3
Fig. 4
Here is a toy I happened to see the
other day, which will, I think, serve to illustrate our subject very
well. That toy ought to lie something in this manner
(FIG. 5), and would do so if it were uniform
in substance; but you see it does not; it will get up again. And now
philosophy comes to our aid, and I am perfectly sure, without looking
inside the figure, that there is some arrangement by which the centre
of gravity is at the lowest point when the image is standing upright;
and we may be certain, when I am tilting it over (see FIG. 6),
that I am lifting up the centre of gravity (a), and raising it
from the earth. All this is effected by putting a piece of lead
inside the lower part of the image, and making the base of large
curvature, and there you have the whole secret. But what will happen
if I try to make the figure stand upon a sharp point? You observe I
must get that point exactly under the centre of gravity, or it will
fall over thus [endeavoring unsuccessfully to balance it]; and this,
you see, is a difficult matter; I can not make it stand steadily; but
if I embarrass this poor old lady with a world of trouble, and hang
this wire with bullets at each end about her neck, it is very evident
that, owing to there being those balls of lead hanging down on either
side, in addition to the lead inside, I have lowered the centre of
gravity, and now she will stand upon this point (FIG. 7),
and, what is more, she proves the truth of our philosophy by standing
sideways.
Fig. 5
Fig. 6
Fig. 7
I remember an experiment which puzzled
me very much when a boy. I read it in a conjuring book, and this was
how the problem was put to us: “How,” as the book said, “how to
hang a pail of water, by means of a stick, upon the side of a table”
(FIG. 8). Now I have here a table, a piece
of stick, and a pail, and the proposition is, how can that pail be
hung to the edge of this table? It is to be done, and can you at all
anticipate what arrangement I shall make to enable me to succeed? Why
this. I take a stick, and put it in the pail between the bottom and
the horizontal piece of wood, and thus give it a stiff handle, and
there it is; and, what is more, the more water I put into the pail,
the better it will hang. It is very true that before I quite
succeeded I had the misfortune to push the bottoms of several pails
out; but here it is hanging firmly (FIG. 9),
and you now see how you can hang up the pail in the way which the
conjuring books require.
Fig. 8
Fig. 9
Again, if you are really so inclined
(and I do hope all of you are), you will find a great deal of
philosophy in this [holding up a cork and a pointed thin stick about
a foot long]. Do not refer to your toy-books, and say you have seen
that before. Answer me rather, if I ask, have you understood it
before? It is an experiment which appeared very wonderful to me when
I was a boy. I used to take a piece of cork (and I remember I thought
at first that it was very important that it should be cut out in the
shape of man, but by degrees I got rid of that idea), and the problem
was to balance it on the point of a stick. Now you will see I have
only to place two sharp-pointed sticks one each side, and give it
wings, thus, and you will find this beautiful condition fulfilled.
Fig. 10
We come now to another point. All
bodies, whether heavy or light, fall to the earth by this force which
we call gravity. By observation, moreover, we see that bodies do not
occupy the same time in falling; I think you will be able to see that
this piece of paper and that ivory ball fall with different
velocities to the table [dropping them]; and if, again, I take a
feather and an ivory ball, and let them fall, you see they reach the
table or earth at different times; that is to say, the ball falls
faster than the feather. Now that should not be so, for all bodies do
fall equally fast to the earth. There are one or two beautiful points
included in that statement. First of all, it is manifest that an
ounce, or a pound, or a ton, or a thousand tons, all fall equally
fast, no one faster than another: here are two balls of lead, a very
light one and a very heavy one, and you perceive they both fall to
the earth in the same time. Now if I were to put into a little bag a
number of these balls sufficient to make up a bulk equal to the large
one, they would also fall in the same time; for it an avalanche fall
from the mountains, the rocks, snow, and ice, together falling toward
the earth, fall with the same velocity, whatever be their size.
I can not take a better illustration
of this than of gold leaf, because it brings before us the reason of
this apparent difference in the time of the fall. Here is a piece of
gold leaf. Now if I take a lump of gold and this gold leaf, and let
them fall through the air together, you see that the lump of gold—the
sovereign or coin—will fall much faster than the gold leaf. But
why? They are both gold, whether sovereign or gold leaf. Why should
they not fall to the earth with the same quickness? They
would do so, but that the air around our globe interferes
very much where we have the piece of gold so extended and enlarged as
to offer much obstruction on falling through it. It will, however,
show you that gold leaf does fall as fast when the
resistance of the air is excluded; for if I take a piece of gold leaf
and hang it in the centre of a bottle so that the gold, and the
bottle, and the air within shall all have an equal chance of falling,
then the gold leaf will fall as fast as anything else. And if I
suspend the bottle containing the gold leaf to a string, and set it
oscillating like a pendulum, I may make it vibrate as hard as I
please and the gold leaf will not be disturbed, but will swing as
steadily as a piece of iron would do; and I might even swing it round
my head with any degree of force, and it would remain undisturbed. Or
I can try another kind of experiment: if I raise the gold leaf in
this way [pulling the bottle up to the ceiling of the theatre by
means of a cord and pulley, and then suddenly letting it fall within
a few inches of the lecture table], and allow it then to fall from
the ceiling downward (I will put something beneath to catch it,
supposing I should be maladroit), you will perceive that
the gold leaf is not in the least disturbed. The resistance of the
air having been avoided, the glass bottle and gold leaf all fall
exactly in the same time.
Here is another illustration: I have
hung a piece of gold leaf in the upper part of this long glass
vessel, and I have the means by a little arrangement at the top, of
letting the gold leaf loose. Before we let it loose we will remove
the air by means of an air-pump, and, while that is being done, let
me show you another experiment of the same kind. Take a penny—piece,
or a half crown, and a round piece of paper a trifle smaller in
diameter than the coin, and try them side by side to see whether they
fall at the same time [dropping them]. You see they do not—the
penny-piece goes down first. But, not place this paper flat on the
top of the coin, so that it shall not meet with any resistance from
the air, and upon then dropping them you see they do both
fall in the same time [exhibiting the effect]. I dare say, if I were
to put this piece of gold leaf, instead of the paper, on the coin, it
would do as well. It is very difficult to lay the gold leaf so flat
that the air shall not get under it and lift it up in falling, and I
am rather doubtful as to the success of this, because the gold leaf
is puckery, but will risk the experiment. There they go together!
[letting them fall] and you see at once that they both reach the
table at the same moment.
We have now pumped the air out of the
vessel, and you will perceive that the gold leaf will fall as quickly
in this vacuum as the coin does in the air. I am now going to let it
loose, and you must watch to see how rapidly it falls. There!
[letting the gold loose]. there it is, falling as gold should fall.
I am sorry to see our time for parting
is drawing so near. As we proceed, I intend to write upon the board
behind me certain words, so as to recall to your minds what we have
already examined; and I put the word FORCES as a heading, and I will
then add beneath the names of the special forces according to the
order in which we consider them; and although I fear that I have not
sufficiently pointed out to you the more important circumstances
connected with the force of GRAVITATION, especially
the law which governs its attraction (for which, I think, I must take
up a little time at our next meeting), still I will put that word on
the board, and hope you will now remember that we have in some degree
considered the force of gravitation—that force which
causes all bodies to attract each other when they are at sensible
distances apart, and tends to draw them together.
Note 1. Add a little
liquid to the marble and decompose it. Marble is composed
of carbonic acid and lime, and, in chemical
language, is called carbonate of lime. When sulphuric acid
is added to it, the carbonic acid is set free, and the sulphuric acid
unites with the lime to form sulphate of lime.
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