3.2 Michael Faraday

And an Introduction to Fields

Normally, new Fields & Energy posts appear on Wednesday mornings. And yet, here we are, a day early, on Tuesday March 12, 2024. Why? Because today is the 192^nd anniversary of what is arguably the single most important day in electromagnetic history. What happened on March 12, 1832? And why don’t more people know about it?

Read on!

Immediately after the results of Örsted and Ampère demonstrated that electric current was magnetic, many scientists began seeking the opposite: evidence that a magnet could produce electrical effects. These efforts proved fruitless. Through use of his extraordinary experimental talent, guided by his physical conception of lines of electric and magnetic force, Michael Faraday (1791–1867) made the crucial discovery in 1831: that just as the moving electrical charges in a current produce magnetic effects, so also do moving magnets produce electrical effects [[i], [ii]]. The connection between electric and magnetic effects manifests itself only in moving or dynamic situations, not in stationary or static situations. This process whereby a change in the magnetic field induces an electric field is called induction. Faraday not only discovered electromagnetic induction, but also he was the first to put it to practical use: through his invention of electric generators (or dynamos), transformers, and electric motors.

Figure 3.4: Michael Faraday (1791–1867) pioneered the modern theory of electricity and magnetism with his insight that electric and magnetic effects propagate through space [[vi]].

Born near London to a humble background and almost entirely self-taught, Michael Faraday stands among the greatest experimental physicists ever. Faraday did not have the academic background necessary to take advantage of the latest mathematical discoveries. His education comprised reading the pages he handled as an apprenticed book binder. Maxwell pointed out, “Open Poisson and Ampère, who went before him, or Weber and Neumann, who came after him, and you will find their pages full of symbols, not one of which Faraday would have understood” [[iii]]. Faraday of necessity developed his own common-sense methods.

The patterns iron filings form in the presence of a magnet have fascinated students since the Middle Ages and probably earlier. Similar patterns may be formed by electrically charged objects [[iv]]. To Faraday, these patterns suggested that the space about a magnet, for instance, is permeated by “lines of magnetic force” [[v]]. These lines represent the direction of the magnetic intensity at any location about the magnet – the direction in which a small compass needle will align due to the torque exerted by the magnetic field. The relative density of the lines indicates the areas where the magnetic effect is most intense.

Figure 3.5: Faraday’s observation of the pattern of iron filings around a bar magnet [[vii]].

His idea of “lines of force” followed directly from the phenomena of nature – his direct observations and analogies to ripples on a pond or tensions in an elastic material. By developing and applying such physical conceptions, Faraday made his great discoveries.

Faraday did not regard lines of force as merely an abstraction, however. He believed his lines of force corresponded to actual physical behavior.

...I cannot refrain from again expressing my conviction of the truthfulness of the representation, which the idea of lines of force affords in regard to magnetic action. All the points which are experimentally established with regard to that action, i.e. all that is not hypothetical, appear to be well and truly represented by it [[viii]].

Faraday’s field lines were not just a conceptual tool, but also an experimental tool. He advocated the use of iron filings to make visible the magnetic fields involved in his experiments:

It would be a voluntary and unnecessary abandonment of most valuable aid if an experimentalist who chooses to consider magnetic power as represented by lines of magnetic force were to deny himself the use of iron filings. By their employment, he may make many conditions of the power, even in complicated cases, visible to the eye at once, may trace the varying direction of the lines of force and determine the relative polarity, may observe in which direction the power is increasing or diminishing, and in complex systems may determine the neutral points, or places where there is neither polarity nor power, even when they occur in the midst of powerful magnets. By their use probable results may be seen at once, and many a valuable suggestion gained for future leading experiments [[ix]].

Faraday generated and preserved such figures by coating paper with molten wax, sprinkling iron filings, applying a magnetic field, and then allowing the wax to cool and freeze the pattern of filings [[x]]. Using this technique, he investigated many of the most common magnetic field configurations studied by beginning students of magnetism today.

Figure 3.6: Iron filings (a) and field pattern (b) around a wire cross-section, and iron filings (c) and field pattern (d) around a current loop. [xi]

Figure 3.6 shows Faraday’s results for some simple configurations: the circular magnetic field lines (H) looping around a wire carrying a current (I), and the magnetic field lines through a loop of current, presented in cross-section.

Figure 3.7: Faraday’s iron filing observations [[xii]], above, and analytic depiction of the fields [[xiii]], below. Note the cases are reversed, between top and bottom.

Faraday also used his approach to investigate new physics, like the behavior of paramagnetic objects (P), which tend to concentrate magnetic fields, and diamagnetic objects (D), which tend to repel magnetic fields. Figure 3.7 shows some of his actual experimental results (top), as well as his idealized representation of the field line behavior (bottom). Faraday’s iron filings embedded in wax captured the magnetic field patterns he investigated and preserved them for posterity. We’ll have more to say about Faraday’s beautiful results, how they illustrate something called the “right-hand rule,” and what these pictures mean for how magnetic fields work later in this chapter.

Maxwell compared and contrasted the work of Ampère with that of Faraday:

The method of Ampère, however, though cast into an inductive form, does not allow us to trace the formation of the ideas which guided it. We can scarcely believe that Ampère really discovered the law of action by means of the experiments which he describes. We are led to suspect, what, indeed, he tells us himself, that he discovered the law by some process which he has not shewn us, and that when he had afterwards built up a perfect demonstration he removed all traces of the scaffolding by which he had raised it.

Faraday, on the other hand, shews us his unsuccessful as well as his successful experiments, and his crude ideas as well as his developed ones, and the reader, however inferior to him in inductive power, feels sympathy even more than admiration, and is tempted to believe that, if he had the opportunity, he too would be a discoverer. Every student therefore should read Ampère's research as a splendid example of scientific style in the statement of a discovery, but he should also study Faraday for the cultivation of a scientific spirit, by means of the action and reaction which will take place between newly discovered facts and nascent ideas in his own mind [[xiv]].

In 1832, with the mainstream of physical thought still firmly in the grip of action-at-a-distance thinking, and while waiting for the paper with his 1831 discoveries to complete review, Faraday had a remarkable insight. Magnetic fields formed circles or loops around conductors. In the event of a change in the magnetic state, wouldn’t those loops ripple out from the source in the same way water waves ripple out from a pebble dropped onto the smooth surface of a pond? Faraday didn’t have the full conception of a true electromagnetic wave which would require a combination of both an electric and a magnetic wave. Yet this 1832 epiphany was the original conception of disturbances in fields rippling away from the sources which created them. Wanting to maintain his priority and preserve an impartial record of his thoughts, Faraday wrote a secret letter and caused it to be deposited in the Strong Box at the Royal Society in London [[xv]].

Royal Institution
March 12, 1832

Certain of the results of the investigations which are embodied in the two papers entitled ‘Experimental Researches in Electricity’ lately read to the Royal Society, and the views arising therefrom, in connexion with other views and experiments lead me to believe that magnetic action is progressive, and requires time, i.e. that when a magnet acts upon a distant magnet or piece of iron, the influencing cause (which I may for the moment call magnetism) proceeds gradually from the magnetic bodies, and requires time for its transmission, which will probably be found to be very sensible.

I think also, that I see reason for supposing that electric induction (of tension) is also performed in a similar progressive way.

I am inclined to compare the diffusion of magnetic forces from a magnetic pole to the vibrations upon the surface of disturbed water, or those of air in the phenomenon of sound; i.e. I am inclined to think the vibratory theory will apply to these phenomena as it does to sound, and most probably to light.

By analogy, I think it may possibly apply to the phenomenon of induction of electricity of tension also.

These views I wish to work out experimentally; but as much of my time is engaged in the duties of my office, and as the experiments will therefore be prolonged, and may in their course be subject to the observation of others, I wish, by depositing this paper in the care of the Royal Society, to take possession as it were of a certain date; and so have right, if they are confirmed by experiment, to claim credit for the views at that date; at which time as far as I know, no one is conscious of or can claim them but myself.

M. Faraday

The modest man of science gave his letter to the Secretary of the Royal Society of London and went to his grave 35 years later, never retrieving his letter to demonstrate his foresight to his peers. The letter lay forgotten in the Strong Box for over a century, only coming to light when Sir William Bragg (1862–1942) opened it on June 24, 1937 [[xvi]]. Although the letter had no influence on the development of electromagnetism, this remarkable time capsule confirms Faraday’s prescience.

A “field” is a physical property that can be ascribed to any point in a region of space. For instance, we could define a temperature field in a room, describing how the temperature varies mathematically from floor to ceiling or from front to back. This approach assigns a temperature to each point in the room. Faraday’s “lines of force” give the intensity and direction of the magnetic influence at each point in space, so his was a “field theory” of magnetism. Faraday was skeptical of physical atoms, but welcomed Boscovich’s point-particle interaction hypothesis as a model of how the æther works [[xvii], [xviii]].

Before Faraday, physicists thought in terms of mathematical relationships governing physical behavior through instantaneous action at a distance. Faraday’s key insight was that electric and magnetic effects have physical causes and these causes can be described as fields permeating with a finite velocity of propagation throughout space. Although it would await Maxwell’s genius to mathematically describe Faraday’s conception, Faraday’s forgotten letter shows that he foresaw the complete picture of fields evolving and propagating across space at least a decade before any of his contemporaries. Faraday’s 1846 paper, “Thoughts on Ray Vibrations,” lays out his more mature thinking [[xix]].

Perhaps no physicist in modern times better lived up to Aristotle’s standard of an intimate association with nature and its phenomena than did Michael Faraday. His commonsense conclusions about the nature of electricity and magnetism are the basis of the modern field theory of electromagnetism. A conceptual innovator, his concepts were always suggested by and followed from masterful examinations of nature’s behavior. Faraday’s physics was clearly Aristotelian.

Next time, 3.3 Potentials & Analysis.

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References

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[i] Anderson, R. (1993). The referees’ assessment of Faraday’s electromagnetic induction paper of 1831. Notes and Records of the Royal Society of London, 47(2), 243–256. doi:10.1098/rsnr.1993.0031 

[[ii]]        Al-Khalili, J. (2015). The birth of the electric machines: a commentary on Faraday (1832) “Experimental researches in electricity.” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 373(2039), 20140208–20140208. doi:10.1098/rsta.2014.0208

[[iii]]       James Clerk Maxwell, “Faraday,” Nature, 8, reprinted in  The Scientific Papers of James Clerk Maxwell  (W.D. Niven, ed.), (New York:  Dover, 1953?) pp. 359.  Originally published, 1890.

[[iv]        Oleg Jefimenko’s EM book has such electric field diagrams.

[[v]]        James Clerk Maxwell, "On Action at a Distance," Proceedings of the Royal Institution of Great Britain, Vol. VII, reprinted in  The Scientific Papers of James Clerk Maxwell  (W.D. Niven, ed.), (New York:  Dover, 1953?) pp. 318.  Originally published, 1890.
See: https://books.google.com/books/content?id=lzlRAAAAYAAJ&pg=PA319&img=1&zoom=3&hl=en&sig=ACfU3U1MSZKms0ZwulC0ZWqbKIO95YAQZA&ci=117%2C262%2C701%2C260&edge=0

[[vi]]        https://commons.wikimedia.org/wiki/File:Michael_Faraday_-_Project_Gutenberg_eText_13103.jpg, Source:      The Project Gutenberg eBook, Great Britain and Her Queen, by Anne E. Check the UWB antenna book reference.

[[vii]]       Faraday, Michael, Experimental Researches in Electricity, Vol. III, New York:  Dover Publications, Inc., 1965, Plate IV.  Originally published 1855. Formerly: Review of Faraday and the Electrical Sciences, The Civil Engineer and Architect’s Journal, Vol. VII April 1844, p. 168. See: https://archive.org/stream/civilengineerarc07lond/#page/168/mode/1up

[[viii]]      Faraday, Michael, Experimental Researches in Electricity, Vol. III, § 3174 (New York:  Dover Publications, Inc., 1965) p. 368.  Originally published 1855.

[[ix][       Faraday, Michael, Experimental Researches in Electricity, Vol. III, § 3234, New York:  Dover Publications, Inc., 1965, p. 397.  Originally published 1855.

[[x]]        See, for instance: https://www.rigb.org/our-history/iconic-objects/iconic-objects-list/faradays-iron-filings

[[xi]]       Faraday, Michael, Experimental Researches in Electricity, Vol. III, New York:  Dover Publications, Inc., 1965, Plate III. Originally published 1855.

[[xii]]       Faraday, Michael, Experimental Researches in Electricity, Vol. III, New York:  Dover Publications, Inc., 1965, Plate III. Originally published 1855.

[[xiii]]      Faraday, Michael, Experimental Researches in Electricity, Vol. III, § 2821, New York:  Dover Publications, Inc., 1965, p. 208.  Originally published 1855.

[[xiv]]      Maxwell, James Clerk, A Treatise on Electricity and Magnetism, vol. 2, 1^st ed. Oxford: At the Clarendon Press, 1873, pp. 162-163.

[[xv]]       Garratt, G.R.M., The Early History of Radio, London: Institute of Electrical Engineers, 1994, p. 10. THAT was serendipitous! The evening before this posted, I came across the same story in William Berkson’s Fields of Force, New York: John Wiley, 1974, p. 73. See: https://amzn.to/3wMNywx. Alternatively, I found a used copy for less than $30 on AbeBooks: https://www.abebooks.com/servlet/SearchResults?sts=t&ref_=search_f_hp&tn=Fields%20of%20Force&an=William%20Berkson. Berkson cites L. Pearce William’s Faraday, 1965, p. 181. Used copies are available on AbeBooks. Thanks to David at Engineering Reality for the tip.

[[xvi]]      Moss, John, “An inductive genius,” New Scientist, November 5, 1981, p. 393.

[[xvii]]      Faraday, Michael, “A speculation touching Electric Conduction and the Nature of Matter, To Richard Taylor, Esq.,” Jan. 25, 1844. In Experimental Researches in Electricity, London: Richard and John Edward Taylor, vol.II, 1844, pp.284-293.

[[xviii]]     Plaice, John, “The Point-atoms of Ruđer Josip Bošković: The Key to the Fields of Michael Faraday,” Fiat Lux, January 2, 2024. See:

The Point-atoms of Ruđer Josip Bošković: The Key to the Fields of Michael Faraday

[[xix]]      Faraday, Michael, “Thoughts on Ray-Vibration,” May 1846, reprinted in Experimental Researches in Electricity, Vol. 3, London: Bernard Quaritch, 15 Piccadilly, 1855, pp. 447‑452 See: http://bit.ly/VZaD8g.