Faraday as a Discoverer - John Tyndall (children's ebooks online txt) 📗

phenomena was certainly in his thoughts. ‘How the magnetic force,’ he writes, ‘is transferred through bodies or through space we know not; whether the result is merely action at a distance, as in the case of gravity; or by some intermediate agency, as in the case of light, heat, the electric current, and (as I believe) static electric action.

The idea of magnetic fluids, as applied by some, or of Magnetic centres of action, does not include that of the latter kind of transmission, but the idea of lines of force does.’ And he continues thus:—

‘I am more inclined to the notion that in the transmission of the [magnetic] force there is such an action [an intermediate agency]

external to the magnet, than that the effects are merely attraction and repulsion at a distance. Such an affection may be a function of the ether; for it is not at all unlikely that, if there be an ether, it should have other uses than simply the conveyance of radiations.’

When he speaks of the magnet in certain cases, ‘revolving amongst its own forces,’ he appears to have some conception of this kind in view.

 

A great part of the investigation completed in October, 1851, was taken up with the motions of wires round the poles of a magnet and the converse. He carried an insulated wire along the axis of a bar magnet from its pole to its equator, where it issued from the magnet, and was bent up so as to connect its two ends. A complete circuit, no part of which was in contact with the magnet, was thus obtained.

He found that when the magnet and the external wire were rotated together no current was produced; whereas, when either of them was rotated and the other left at rest currents were evolved. He then abandoned the axial wire, and allowed the magnet itself to take its place; the result was the same.[2] It was the relative motion of the magnet and the loop that was effectual in producing a current.

 

The lines of force have their roots in the magnet, and though they may expand into infinite space, they eventually return to the magnet.

Now these lines may be intersected close to the magnet or at a distance from it. Faraday finds distance to be perfectly immaterial so long as the number of lines intersected is the same.

For example, when the loop connecting the equator and the pole of his barmagnet performs one complete revolution round the magnet, it is manifest that all the lines of force issuing from the magnet are once intersected. Now it matters not whether the loop be ten feet or ten inches in length, it matters not how it may be twisted and contorted, it matters not how near to the magnet or how distant from it the loop may be, one revolution always produces the same amount of current electricity, because in all these cases all the lines of force issuing from the magnet are once intersected and no more.

 

From the external portion of the circuit he passes in idea to the internal, and follows the lines of force into the body of the magnet itself. His conclusion is that there exist lines of force within the magnet of the same nature as those without. What is more, they are exactly equal in amount to those without. They have a relation in direction to those without; and in fact are continuations of them…. ‘Every line of force, therefore, at whatever distance it may be taken from the magnet, must be considered as a closed circuit, passing in some part of its course through the magnet, and having an equal amount of force in every part of its course.’

 

All the results here described were obtained with moving metals.

‘But,’ he continues with profound sagacity, ‘mere motion would not generate a relation, which had not a foundation in the existence of some previous state; and therefore the quiescent metals must be in some relation to the active centre of force,’ that is to the magnet.

He here touches the core of the whole question, and when we can state the condition into which the conducting wire is thrown before it is moved, we shall then be in a position to understand the physical constitution of the electric current generated by its motion.

 

In this inquiry Faraday worked with steel magnets, the force of which varies with the distance from the magnet. He then sought a uniform field of magnetic force, and found it in space as affected by the magnetism of the earth. His next memoir, sent to the Royal Society, December 31, 1851, is ‘on the employment of the Induced Magnetoelectro Current as a test and measure of magnetic forces.’

He forms rectangles and rings, and by ingenious and simple devices collects the opposed currents which are developed in them by rotation across the terrestrial lines of magnetic force. He varies the shapes of his rectangles while preserving their areas constant, and finds that the constant area produces always the same amount of current per revolution. The current depends solely on the number of lines of force intersected, and when this number is kept constant the current remains constant too. Thus the lines of magnetic force are continually before his eyes, by their aid he colligates his facts, and through the inspirations derived from them he vastly expands the boundaries of our experimental knowledge. The beauty and exactitude of the results of this investigation are extraordinary. I cannot help thinking while I dwell upon them, that this discovery of magneto-electricity is the greatest experimental result ever obtained by an investigator. It is the Mont Blanc of Faraday’s own achievements. He always worked at great elevations, but a higher than this he never subsequently attained.

 

Footnotes to Chapter 13

 

[1] He compares the interpenetration of two atoms to the coalescence of two distinct waves, which though for a moment blended to a single mass, preserve their individuality, and afterwards separate.

 

[2] In this form the experiment is identical with one made twenty years earlier. See page 34.

 

Chapter 14.

 

Unity and convertibility of natural forces: theory of the electric current.

 

The terms unity and convertibility, as applied to natural forces, are often employed in these investigations, many profound and beautiful thoughts respecting these subjects being expressed in Faraday’s memoirs. Modern inquiry has, however, much augmented our knowledge of the relationship of natural forces, and it seems worth while to say a few words here, tending to clear up certain misconceptions which appear to exist among philosophic writers regarding this relationship.

 

The whole stock of energy or working-power in the world consists of attractions, repulsions, and motions. If the attractions and repulsions are so circumstanced as to be able to produce motion, they are sources of working-power, but not otherwise. Let us for the sake of simplicity confine our attention to the case of attraction. The attraction exerted between the earth and a body at a distance from the earth’s surface is a source of working-power; because the body can be moved by the attraction, and in falling to the earth can perform work. When it rests upon the earth’s surface it is not a source of power or energy, because it can fall no further. But though it has ceased to be a source of energy, the attraction of gravity still acts as a force, which holds the earth and weight together.

 

The same remarks apply to attracting atoms and molecules. As long as distance separates them, they can move across it in obedience to the attraction, and the motion thus produced may, by proper appliances, be caused to perform mechanical work. When, for example, two atoms of hydrogen unite with one of oxygen, to form water the atoms are first drawn towards each other—they move, they clash, and then by virtue of their resiliency, they recoil and quiver. To this quivering motion we give the name of heat. Now this quivering motion is merely the redistribution of the motion produced by the chemical affinity; and this is the only sense in which chemical affinity can be said to be converted into heat. We must not imagine the chemical attraction destroyed, or converted into anything else.

For the atoms, when mutually clasped to form a molecule of water, are held together by the very attraction which first drew them towards each other. That which has really been expended is the pull exerted through the space by which the distance between the atoms has been diminished.

 

If this be understood, it will be at once seen that gravity may in this sense be said to be convertible into heat; that it is in reality no more an outstanding and inconvertible agent, as it is sometimes stated to be, than chemical affinity. By the exertion of a certain pull, through a certain space, a body is caused to clash with a certain definite velocity against the earth. Heat is thereby developed, and this is the only sense in which gravity can be said to be converted into heat. In no case is the force which produces the motion annihilated or changed into anything else. The mutual attraction of the earth and weight exists when they are in contact as when they were separate; but the ability of that attraction to employ itself in the production of motion does not exist.

 

The transformation, in this case, is easily followed by the mind’s eye. First, the weight as a whole is set in motion by the attraction of gravity. This motion of the mass is arrested by collision with the earth; being broken up into molecular tremors, to which we give the name of heat.

 

And when we reverse the process, and employ those tremors of heat to raise a weight, as is done through the intermediation of an elastic fluid in the steam-engine, a certain definite portion of the molecular motion is destroyed in raising the weight. In this sense, and this sense only, can the heat be said to be converted into gravity, or more correctly, into potential energy of gravity. It is not that the destruction of the heat has created any new attraction, but simply that the old attraction has now a power conferred upon it, of exerting a certain definite pull in the interval between the starting-point of the falling weight and its collision with the earth.

 

So also as regards magnetic attraction: when a sphere of iron placed at some distance from a magnet rushes towards the magnet, and has its motion stopped by collision, an effect mechanically the same as that produced by the attraction of gravity occurs. The magnetic attraction generates the motion of the mass, and the stoppage of that motion produces heat. In this sense, and in this sense only, is there a transformation of magnetic work into heat. And if by the mechanical action of heat, brought to bear by means of a suitable machine, the sphere be torn from the magnet and again placed at a distance, a power of exerting a pull through that distance, and producing a new motion of the sphere, is thereby conferred upon the magnet; in this sense, and in this sense only, is the heat converted into magnetic potential energy.

 

When, therefore, writers on the conservation of energy speak of tensions being ‘consumed’ and ‘generated,’ they do not mean thereby that old attractions have been annihilated and new ones brought into existence, but that, in the one case, the power of the attraction to produce motion has been diminished by the shortening of the distance between the attracting bodies, and that in the other case the power of producing motion has been augmented by the increase of the distance. These remarks apply to all bodies, whether they be