On the various forces of nature and their relations to each other

Michael Faraday

On the various forces of nature and their relations to each other

PREFACE

Which was first, Matter or Force? If we think on this question, we shall find that we are unable to conceive of matter without force, or of force without matter. When God created the elements of which the earth is composed, He created certain wondrous forces, which are set free, and become evident when matter acts on matter. All these forces, with many differences, have much in common, and if one is set free, it will immediately endeavour to free its companions. Thus, heat will enable us to eliminate light, electricity, magnetism, and chemical action; chemical action will educe light, electricity, and heat. In this way we find that all the forces in nature tend to form mutually dependent systems; and as the motion of one star affects another, so force in action liberates and renders evident forces previously tranquil.

We say tranquil, and yet the word is almost without meaning in the Cosmos. – Where do we find tranquillity? The sea, the seat of animal, vegetable, and mineral changes, is at war with the earth, and the air lends itself to the strife. The globe, the scene of perpetual intestine change, is, as a mass, acting on, and acted on, by the other planets of our system, and the very system itself is changing its place in space, under the influence of a known force springing from an unknown centre.

For many years the English public had the privilege of listening to the discourses and speculations of Professor Faraday, at the Royal Institution, on Matter and Forces; and it is not too much to say that no lecturer on Physical Science, since the time of Sir Humphrey Davy, was ever listened to with more delight. The pleasure which all derived from the expositions of Faraday was of a somewhat different kind from that produced by any other philosopher whose lectures we have attended. It was partially derived from his extreme dexterity as an operator: with him we had no chance of apologies for an unsuccessful experiment – no hanging fire in the midst of a series of brilliant demonstrations, producing that depressing tendency akin to the pain felt by an audience at a false note from a vocalist. All was a sparkling stream of eloquence and experimental illustration. We would have defied a chemist loving his science, no matter how often he might himself have repeated an experiment, to feel uninterested when seeing it done by Faraday.

The present publication presents one or two points of interest. In the first place, the Lectures were especially intended for young persons, and are therefore as free as possible from technicalities; and in the second place, they are printed as they were spoken, verbatim et literatim. A careful and skilful reporter took them down; and the manuscript, as deciphered from his notes, was subsequently most carefully corrected by the Editor as regards any scientific points which were not clear to the short-hand writer; hence all that is different arises solely from the impossibility, alas! of conveying the manner as well as the matter of the Lecturer.

May the readers of these Lectures derive one-tenth of the pleasure and instruction from their perusal which they gave to those who had the happiness of hearing them!

W. CROOKES.

LECTURE I.

THE FORCE OF GRAVITATION

It grieves me much to think that I may have been a cause of disturbance in your Christmas arrangements1, for nothing is more satisfactory to my mind than to perform what I undertake; but such things are not always left in our own power, and we must submit to circumstances as they are appointed. I will to-day do my best, and will ask you to bear with me if I am unable to give more than a few words; and as a substitute, I will endeavour to make the illustrations of the sense I try to express as full as possible; and if we find by the end of this lecture that we may be justified in continuing them, thinking that next week our power shall be greater, – why, then, with submission to you, we will take such course as you may think fit, – either to go on, or discontinue them; and although I now feel much weakened by the pressure of illness (a mere cold) upon me, both in facility of expression and clearness of thought, I shall here claim, as I always have done on these occasions, the right of addressing myself to the younger members of the audience. And for this purpose, therefore, unfitted as it may seem for an elderly infirm man to do so, I will return to second childhood and become, as it were, young again amongst the young.

Let us now consider, for a little while, how wonderfully we stand upon this world. Here it is we are born, bred, and live, and yet we view these things with an almost entire absence of wonder to ourselves respecting the way in which all this happens. So small, indeed, is our wonder, that we are never taken by surprise; and I do think that, to a young person of ten, fifteen, or twenty years of age, perhaps the first sight of a cataract or a mountain would occasion him more surprise than he had ever felt concerning the means of his own existence, – how he came here; how he lives; by what means he stands upright; and through what means he moves about from place to place. Hence, we come into this world, we live, and depart from it, without our thoughts being called specifically to consider how all this takes place; and were it not for the exertions of some few inquiring minds, who have looked into these things and ascertained the very beautiful laws and conditions by which we do live and stand upon the earth, we should hardly be aware that there was anything wonderful in it. These inquiries, which have occupied philosophers from the earliest days, when they first began to find out the laws by which we grow, and exist, and enjoy ourselves, up to the present time, have shewn us that all this was effected in consequence of the existence of certain forces, or abilities to do things, or powers, that are so common that nothing can be more so; for nothing is commoner than the wonderful powers by which we are enabled to stand upright – they are essential to our existence every moment.

It is my purpose to-day to make you acquainted with some of these powers; not the vital ones, but some of the more elementary, and, what we call, physical powers: and, in the outset, what can I do to bring to your minds a notion of neither more nor less than that which I mean by the word power, or force? Suppose I take this sheet of paper, and place it upright on one edge, resting against a support before me (as the roughest possible illustration of something to be disturbed), and suppose I then pull this piece of string which is attached to it. I pull the paper over. I have therefore brought into use a power of doing so – the power of my hand carried on through this string in a way which is very remarkable when we come to analyse it; and it is by means of these powers conjointly (for there are several powers here employed) that I pull the paper over. Again, if I give it a push upon the other side, I bring into play a power, but a very different exertion of power from the former; or, if I take now this bit of shell-lac [a stick of shell-lac about 12 inches long and 1½ in diameter] and rub it with flannel, and hold it an inch or so in front of the upper part of this upright sheet, the paper is immediately moved towards the shell-lac, and by now drawing the latter away, the paper falls over without having been touched by anything. You see – in the first illustration I produced an effect than which nothing could be commoner – I pull it over now, not by means of that string or the pull of my hand, but by some action in the shell-lac. The shell-lac, therefore, has a power wherewith it acts upon the sheet of paper; and as an illustration of the exercise of another kind of power, I might use gunpowder with which to throw it over.

Now, I want you to endeavour to comprehend that when I am speaking of a power or force, I am speaking of that which I used just now to pull over this piece of paper. I will not embarrass you at present with the name of that power, but it is clear there was a something in the shell-lac which acted by attraction, and pulled the paper over; this, then, is one of those things which we call power, or force; and you will now be able to recognise it as such in whatever form I shew it to you. We are not to suppose that there are so very many different powers; on the contrary, it is wonderful to think how few are the powers by which all the phenomena of nature are governed. There is an illustration of another kind of power in that lamp; there is a power of heat – a power of doing something, but not the same power as that which pulled the paper over: and so, by degrees, we find that there are certain other powers (not many) in the various bodies around us. And thus, beginning with the simplest experiments of pushing and pulling, I shall gradually proceed to distinguish these powers one from the other, and compare the way in which they combine together. This world upon which we stand (and we have not much need to travel out of the world for illustrations of our subject; but the mind of man is not confined like the matter of his body, and thus he may and does travel outwards; for wherever his sight can pierce, there his observations can penetrate) is pretty nearly a round globe, having its surface disposed in a manner of which this terrestrial globe by my side is a rough model; so much is land and so much is water, and by looking at it here we see in a sort of map or picture how the world is formed upon its surface. Then, when we come to examine further, I refer you to this sectional diagram of the geological strata of the earth, in which there is a more elaborate view of what is beneath the surface of our globe. And when we come to dig into or examine it (as man does for his own instruction and advantage, in a variety of ways), we see that it is made up of different kinds of matter, subject to a very few powers, and all disposed in this strange and wonderful way, which gives to man a history – and such a history – as to what there is in those veins, in those rocks, the ores, the water springs, the atmosphere around, and all varieties of material substances, held together by means of forces in one great mass, 8,000 miles in diameter, that the mind is overwhelmed in contemplation of the wonderful history related by these strata (some of which are fine and thin like sheets of paper), – all formed in succession by the forces of which I have spoken.

I now shall try to help your attention to what I may say by directing, to-day, our thoughts to one kind of power. You see what I mean by the term matter– any of these things that I can lay hold of with the hand, or in a bag (for I may take hold of the air by enclosing it in a bag) – they are all portions of matter with which we have to deal at present, generally or particularly, as I may require to illustrate my subject. Here is the sort of matter which we call water, – it is there ice [pointing to a block of ice upon the table], there water [pointing to the water boiling in a flask], here vapour – you see it issuing out from the top [of the flask]. Do not suppose that that ice and that water are two entirely different things, or that the steam rising in bubbles and ascending in vapour there is absolutely different from the fluid water. It may be different in some particulars, having reference to the amounts of power which it contains; but it is the same, nevertheless, as the great ocean of water around our globe, and I employ it here for the sake of illustration, because if we look into it we shall find that it supplies us with examples of all the powers to which I shall have to refer. For instance, here is water – it is heavy; but let us examine it with regard to the amount of its heaviness, or its gravity. I have before me a little glass vessel and scales [nearly equipoised scales, one of which contained a half-pint glass vessel], and the glass vessel is at present the lighter of the two; but if I now take some water and pour it in, you see that that side of the scales immediately goes down; that shews you (using common language, which I will not suppose for the present you have hitherto applied very strictly) that it is heavy: and if I put this additional weight into the opposite scale, I should not wonder if this vessel would hold water enough to weigh it down. [The Lecturer poured more water into the jar, which again went down.] Why do I hold the bottle above the vessel to pour the water into it? You will say, because experience has taught me that it is necessary. I do it for a better reason – because it is a law of nature that the water should fall towards the earth, and therefore the very means which I use to cause the water to enter the vessel are those which will carry the whole body of water down. That power is what we call gravity, and you see there [pointing to the scales] a good deal of water gravitating towards the earth. Now here [exhibiting a small piece of platinum2] is another thing which gravitates towards the earth as much as the whole of that water. See what a little there is of it —that little thing is heavier than so much water [placing the metal in opposite scales to the water]. What a wonderful thing it is to see that it requires so much water as that [a half-pint vessel full] to fall towards the earth, compared with the little mass of substance I have here! And again, if I take this metal [a bar of aluminium3 about eight times the bulk of the platinum], we find the water will balance that as well as it did the platinum; so that we get, even in the very outset, an example of what we want to understand by the words forces or powers.

I have spoken of water, and first of all of its property of falling downwards. You know very well how the oceans surround the globe – how they fall round the surface, giving roundness to it, clothing it like a garment; but, besides that, there are other properties of water. Here, for instance, is some quick-lime, and if I add some water to it, you will find another power or property in the water.4 It is now very hot, it is steaming up, and I could perhaps light phosphorus or a lucifer match with it. Now, that could not happen without a force in the water to produce the result; but that force is entirely distinct from its power of falling to the earth. Again, here is another substance [some anhydrous sulphate of copper5] which will illustrate another kind of power. [The Lecturer here poured some water over the white sulphate of copper, which immediately became blue, evolving considerable heat at the same time.] Here is the same water, with a substance which heats nearly as much as the lime does; but see how differently. So great indeed is this heat in the case of lime, that it is sufficient sometimes (as you see here) to set wood on fire; and this explains what we have sometimes heard, of barges laden with quick-lime taking fire in the middle of the river, in consequence of this power of heat brought into play by a leakage of the water into the barge. You see how strangely different subjects for our consideration arise, when we come to think over these various matters, – the power of heat evolved by acting upon lime with water, and the power which water has of turning this salt of copper from white to blue.

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 downwards 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 downwards, to sink it thus in the ball of air.

Fig. 1.

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 balls 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 towards 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.

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 towards bodies which fall to it, but, that all these bodies possess an attraction, every one towards 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 towards it; shewing 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 towards 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.

Fig. 2.

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 shew 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 it6, 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 towards the other, and all fall towards 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 vapour, 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 shewn by introducing into it a lighted taper, which was immediately extinguished.] You see from this result also that it gravitates. All these experiments shew you that, tried by the balance, tried by pouring like water from one vessel to another, this steam, or vapour, or gas, is, like all other things, attracted to the earth.

Fig. 3. and Fig. 4.

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 hold 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 that 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.