”It was the line of force which tied all his researches on electricity and magnetism together...”
FIRST EDITION IN ORIGINAL WRAPPERS of two papers containing Faraday’s detailed investigations of the nature of the ‘lines of force’; an extension of work he had begun in his first paper (1821) on electromagnetism. These investigations laid the foundations of field theory.
“Faraday’s work on electromagnetic rotations led him to take a view of electromagnetism different from that of most of his contemporaries. Where they focused on the electrical fluids and the peculiar forces engendered by their motion (Ampère’s position), he was forced to consider the line of force. He did not know what it was in 1821, but he suspected that it was a state of strain in the molecules of the current carrying wire and the surrounding medium produced by the passage of an electrical ‘current’ (whatever that was) through the wire … It was the line of force which tied all his researches on electricity and magnetism together” (DSB).
“It was not until July of 1851 that Faraday was able to turn his attention fully to the investigation of the intimate nature of lines of force... His purpose was nothing less than to supply a general view of the modes of action of force. Central to this view was the physical reality of the lines of force.
“The basic question to which Faraday turned in the summer of 1851 concerned the interpretation of the pattern made by iron filings sprinkled on a card over a magnet. The filings arranged themselves in lines; were these lines ‘real’ or were they merely the result of the interaction of the magnet and the iron filings? Faraday had long viewed them as strains of some sort but it was now time to discover their true nature. If strains, to what were they connected so that the strain could be imposed along the line of force? The electrostatic line of force was firmly anchored in electrically excited matter and the strain, transmitted along the curves of the intervening polarized particles, ended in positively and negatively charged surfaces. An electrostatic line of force could start in a charged sphere and leap across a room to the wall. If the sphere were positively charged, the part of the wall where the line of force ended would be negative. The line, and the particles in between were all polar having ‘positive’ and ‘negative’ ends. Magnetic lines were peculiar in that they always returned to the body from which they emanated. It was impossible to hold up a sphere ‘charged’ with north magnetism and trace a line of magnetic force across a room to a south pole on the wall. Wherever a north pole existed, a south was also to be found, nearby, in the same body. The ends of the line of force, then, had to be the poles of the magnet. This was where the strain originated; here must be where the original tension was applied.
“When examined critically this explanation made little sense. An iron magnet was, after all, relatively homogeneous. Why, then, should two particular spots, indistinguishable from other places, become poles? Why, to put it another way, should the lines of force terminate at all? From 1845 to 1850 Faraday had gradually convinced himself that the actual particles of magnetic or diamagnetic substances counted for very little in magnetic phenomena. Why, then, call in particles merely to have an anchor for the lines of force? Could not poles be dispensed with altogether?
“The first thing that had to be done was to make certain that the lines of force really existed independently of the iron filings that illustrated their forms so beautifully. Since iron itself was magnetic, it was possible that the magnetic curves might be the result of placing iron filings over a magnet and that when the filings were not present, the curves vanished. The use of a compass needle was open to the same objections. If the lines of force were created by the interaction of the needle and the magnet, the needle would still trace them out as if the lines existed independently of the needle. One method alone appeared free from fault. A conducting wire in the presence of a magnet showed no effect; when the wire was moved across the lines of force, a current was generated. The moving wire involved no attraction, repulsion, or other polar effects. The lines of force detected by this method would, therefore, not appear to be created by the presence of the wire. ‘So,’ Faraday concluded, ‘a moving wire may be accepted as a correct philosophical indication of the presence of magnetic force’ (3083).
“The existence of the lines of force gave no hints about their essential properties. Were they continuous curves, or were they actually attached to points in the magnet called poles? If they were continuous curves, then the lines of force ought to pass through the magnet as well as around it in the external medium. Could these lines be detected inside the magnet? Faraday devised a very simple apparatus for this purpose. Two bar magnets were placed side by side with similar poles next to one another. The two magnets were separated by a thin piece of wood, reaching from the middle of the magnets to one end. The two magnets were then placed in a wooden axle so that they could be rotated about their mutual axis. A copper collar was then placed around the magnets at their middle. A loop of wire could now be arranged so as to make contact with the collar at one end and with a galvanometer at the other. Another wire ran from the galvanometer, down the groove left between the two magnets, and then up to the collar. Each element in the apparatus could be rotated separately; the two magnets around their mutual axis, the wire running down the centre on its axis, and the loop of copper wire around an axis more or less coincident with the extension of the magnetic axis. With this apparatus, Faraday could hope to detect lines of force if they ran through the magnet as well as through the medium in which the magnet was immersed. He first repeated the experiments he had done in 1832 with the rotating magnet to be certain that the lines of force did not rotate with the magnet. ‘No mere rotation of a bar magnet on its axis, produces any induction effect on circuits exterior to it’, he reported. ‘The system of power about the magnet must not be considered as necessarily revolving with the magnet, any more than the rays of light which emanate from the sun are supposed to revolve with the sun’ (3090). The conclusion that the lines of force did not move with the magnet reinforced the idea that they were, in a sense, independent of the magnet. This independence must also exist within the magnet. Such independence now could easily be shown. The power of a magnet could be measured precisely in terms of the current generated in a wire cutting the lines of force. Faraday clearly showed that the current (or, better, in modern terms, the electromotive force) directly proportional to the number of lines cut. When all the lines of force were cut, no matter whether the cut was perpendicular or oblique to the lines, the current in the detecting wire was the same (3109-3114). ‘The quantity of electricity thrown into a current is directly as the amount of curves intersected’ (3113). Knowing this, the existence of the lines of force within the magnet could be determined with great precision. ‘there exists 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, absolutely unchanged in their nature, so far as the experimental test can be applied to 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’ (3116-7).
“The implications … were literally revolutionary. If Faraday were correct and the lines of force did actually exist with the properties he attributed to them, then the whole structure of orthodox electric and magnetic science must come tumbling down. The orthodox theories were founded upon central forces acting inversely as the square of the distance; Faraday's new theory rejected central forces. The polarity that was the necessary complement of central forces had been banished. There was no polarity exclusive of the line of force and even this polarity was an odd one … polarity was the direction of the line of force, and as such, it was a polarity without poles. Since attraction and repulsion must be attraction to or a repulsion from some point (which then could be considered a pole) Faraday explicitly rejected attraction and repulsion as real magnetic phenomena. Not only did his work on magnetic conduction contradict the older forms of attraction and repulsion, but these older ideas were now capable of preventing further progress by blinding men to new approaches. ‘To assume that pointing is always the direct effect of attractive and repulsive forces acting in couples (as in the cases in question, or as in bismuth crystals), is to shut out ideas, in relation to magnetism, which are already applied in the theories of the nature of light and electricity; and the shutting out of such ideas may be an obstruction to the advancement of truth and a defence of wrong assumptions and error’ (3156).
“There is no doubt that Faraday knew exactly how unorthodox he was and that his ideas were bound to meet with opposition. He knew, too, from which quarter the opposition would come. Hence his insistence upon the experimental aspect of his theory. ‘I keep working away at Magnetism,’ he wrote to Schoenbein, ‘whether well or not I will not say. It is at all events to my own satisfaction. Experiments are beautiful things and I quite revel in the making of them. Besides they give one such confidence and, as I suspect that a good many think me somewhat heretical in magnetics or perhaps rather fantastical, I am very glad to have them to fall back upon.’ The mathematical physicist was unlikely to reject the simplicity of the inverse square law for anything so distinctly unmathematical as the lines of force. It was to this point that Faraday addressed himself in what may well be called the credo of the experimentalist. ‘As an experimentalist’, he wrote, ‘I feel bound to let experiment guide me into any train of thought which it may justify; being satisfied that experiment, like analysis, must lead to strict truth if rightly interpreted; and believing also, that it is in its nature far more suggestive of new trains of thought and new conditions of natural power (3159). Experiment and his own theories had led him to the physical reality of the lines of force. It was with considerable hesitancy, however, that he presented his new conclusions on the nature of the lines of force at the end of the Twenty-eighth Series:
Whilst writing this paper I perceive, that, in the late Series of these Researches, Nos. XXV, XXVI, XXVII, I have sometimes used the term lines of force so vaguely, as to leave the reader doubtful whether I intended it as a merely representative idea of the forces, or as the description of the path along which the power was continuously exerted. What I have said in the beginning of this paper … will render that matter clear. I have as yet found no reason to wish any part of those papers altered, except these doubtful expressions; but that will be rectified if it be understood, that, wherever the expression line of force is taken simply to represent the disposition of the forces, it shall have the fullness of that meaning; but that wherever it may seem to represent the idea of the physical mode of transmission of the force, it expresses in that respect the opinion to which I incline at present. The opinion may be erroneous, and yet all that relates or refers to the disposition of the force will remain the same (3175).
“It was not until 1852 that Faraday insisted upon the reality of the lines of force. In his paper ‘On the Physical Character of the Lines of Force’, he informed the reader that ‘I am now about to leave the strict line of reasoning for a time, and enter upon a few speculations respecting the physical character of the lines of force, and the manner in which they may be supposed to be continued through space’ (3243). There can be no doubt that Faraday was firmly convinced that the lines of force were real. The fact that the magnetic force was transmitted along curves, and that these curves were continuous was evidence enough for him. ‘I cannot conceive curved lines of force without the conditions of a physical existence in that immediate space’ (3258). The reality of the physical lines of force was thus established. But this reality immediately raised a new question. How was the magnetic force transmitted through the lines of force? The search for an answer to this question led Faraday to the foundations of field theory” (Pierce Williams, Michael Faraday, pp. 444-450).
This volume contains the 28th and 29th series of Faraday’s remarkable Experimental Researches in Electricity, comprising sections 3070-3176 and 3177-3242, respectively.
In: Philosophical Transactions, Vol. 142, Part I. London: Taylor and Francis, 1852. Quarto (301 x 231 mm), original wrappers; custom cloth box. A little wear to spine and 7cm closed tear to lower part of front hinge. Rare in original wrappers.
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