As I’ve noted in previous posts, one of the fun things about researching historical scientific papers is the unexpected places the investigation can take you. Often a simple search on a straightforward topic will start a chain reaction of increasingly interesting discoveries, comparable to a trip to the grocery store ending up in Machu Picchu. Case in point: I’ve been doing a series of posts about the research of Michael Faraday (see here and here), but I have yet to write about the paper that originally interested me in the subject! Too many other intriguing observations keep getting in the way.
Case in point in case in point: I’ve been looking into Faraday’s contribution to the understanding that light is an electromagnetic wave. That investigation led me to some early work by other researchers on the light/magnetism connection, and led me in turn to a puzzler: how significant and accurate is that earlier research? I don’t have a good answer, so I will pose the questions to the physics/blog community in the post.
In another example of “chewing the textbook cardboard” (h/t Brian and Blake), James Clerk Maxwell (1831-1879) is usually credited as the scientist who “demonstrated that electricity, magnetism and even light are all manifestations of the same phenomenon,” through his 1864 publication of his eponymous set of equations.
This is, unsurprisingly, a bit of an oversimplification. Maxwell made a huge leap forward by demonstrating that the electromagnetic equations supported waves traveling at the speed of light, which he then associated with light itself. However, a rather cursory look at the journals of the 1800s shows that people had been contemplating a connection between light and electromagnetism for at least 50 years before Maxwell!
The most definitive illustration of this relationship was shown by Faraday in 1845, when he showed that light passing through a material will have its polarization rotated by the application of a magnetic field, a phenomenon known as Faraday rotation. I’ll be coming back to discuss Faraday rotation in its own post, if nothing else completely sidetracks me!
Faraday himself cites a number of other, earlier, experiments which seemed to suggest a relationship between light and magnetism, and it is these earlier experiments which puzzle me.
The first of these was performed somewhere around 1812 by Professor Domenico Lino Morichini (also written Morrichini) of Rome. Though I have been unable to find Morichini’s original reference (if there is one), other scientists have given detailed descriptions of the method. In short, Morichini claimed to have magnetized iron by illuminating it with violet light derived from sunlight! From an article by Mr. J. Murray, “On Aphlogistic Phænomena and the Magnetism of Violet Light,” Philosophical Magazine 53 (1819), 268:
There is another very interesting discovery on which I have expressed a degree of surprise, since amply removed. I advert to the communication of magnetic powers to a steel bar by violet light, announced by Dr. Morrichini of Rome. On my return from Naples, I had the pleasure of seeing the Professor. He has succeeded in magnetizing no less than seventy-four steel bars, and was so good as to present me with one so treated. It is attractive of iron filings, and possesses a high polarity. The following is recorded on the wrapper, descriptive of the circumstances which accompanied the experiment. — “Adi 1 7bre 1812. Num. 3, Essuendo il tempo nuvolo ed umido non è seguita l’esperienza, anzi l’ago perdette quella piccola virtù magnetica che avea acquistata il giorno precedente. Avendolo posto alle solite prove il giorno dopo, si è calamitato al solito.”
It is by no means necessary (as Professor Playfair has stated) that the needle rest in the magnetic plane; for Professor Morrichini assured me that he had succeeded with the needle in various positions on the horizontal level, and even vertical and more or less inclined. The bright solar beam admitted by a convenient aperture is received by the prism. The prism is then turned upon its axis so as to insulate the violet light, and the ray is then projected on the needle by means of a lens possessing considerable convexity, and about three inches diameter. For the rest I beg to give you a few abridged extracts from Professor Morrichini’s memoir, entitled, “Secondo Memoria sopra la forza magnetizante del lembo estremo del raggio violetto,” &c. &c. not published. The red ray of the spectrum does not magnetize, nor the light of combustible bodies, inflamed. The violet light of the lunar beam has given in twelve hours magnetic properties more decisive than the solar red in seven hours and a half.
Dr. Morrichini thus concludes, page 32, his interesting memoir: “The chemical and violet rays are never separated, and the intensity of the violet rays may proportionally announce those of the chemical. Terrestrial bodies may absorb from the solar rays the magnetic fluid as they absorb light and caloric, which two fluids are concerned in their decomposition and recomposition. Iron then may be with regard to the magnetic fluid what pyrophorus is with regard to caloric, and natural phosphori with respect to light.” This beautiful discovery may be said to throw a new light upon light.
Here we can see what seems to be a hint of the electromagnetic nature of light, practically stated as such in the last paragraph. However, the research seems too good to be true. Certainly light-sensitive magnets exist, but I have never heard of a material as mundane as steel possessing properties of photoinduced magnetization. One would be tempted to dismiss the experiment entirely, but the results were apparently reproduced by M. Somerville, “On the magnetizing power of the more refrangible solar rays,” Phil. Trans. Roy. Soc. Lond. 116 (1826), 132. After that, I have so far been only able to find one other reference to this work, which suggests that the experiments were later dismissed. Were they dismissed because they were completely wrong, though, or because it turned out the relationship between light and magnetization was not a direct one? For instance, did a heating of the metal allow the Earth’s magnetic field to somehow perform the magnetization?
Faraday seems to have been of the latter opinion; after noting that he has demonstrated a link between light and magnetism, “I think for the first time”, he then refers to Morrichini’s work:
I say, for the first time, because I do not think that the experiments of Morrichini on the production of magnetism by the rays at the violet end of the spectrum prove any such relation. When in Rome with Sir H. Davy in the month of May 1814, I spent several hours at the house of Morrichini, working with his apparatus and under his directions, but could not succeed in magnetising a needle. I have no confidence in the effect as a direct result of the action of the sun’s rays; but think, that when it has occurred it has been secondary, incidental, and perhaps even accidental; a result that might well happen with a needle that was preserved during the whole experiment in a north and south position.
M. Somerville is a good example of how the study of historical papers leads one to unexpected places. See if you can pick out what caught my eye in the heading of Somerville’s paper:
Yes, that’s Mrs. Somerville! Mary Somerville (1780-1872) was a science writer and scientist, and apparently the second woman to gain recognition in the United Kingdom as a scientist, after Caroline Herschel, the sister of the famous astronomer William Herschel. A picture of Mary (via Wikipedia) is below:
Mary was encouraged in her scientific researches by her husband William, and was evidently an extraordinary scientific mind — to gain recognition in science as a woman in the early 1800s is quite remarkable! She wrote a number of popular science books for her era; at least one of these, “On the Connexion of the Physical Sciences“, can be read via Google Books. It is noteworthy to quote a section of her book relevant to the question of light and magnetism:
In light, heat, and electricity, or magnetism, nature has exhibited principles which do not occasion any appreciable change in the weight of bodies, although their presence is manifested by the most remarkable mechanical and chemical action. These agencies are so connected, that there is reason to believe they will ultimately be referred to some one power of a higher order, in conformity with the general economy of the system of the world, where the most varied and complicated effects are produced by a small number of universal laws. These principles penetrate matter in all directions; their velocity is prodigious, and their intensity varies inversely as the squares of the distances. The development of electric currents, as well by magnetic as electric induction, the similarity in their mode of action in a great variety of circumstances, but, above all, the production of the spark from a magnet, the ignition of metallic wires, and chemical decomposition, show that magnetism can no longer be regarded as a separate independent principle. Although the evolution of light and heat during the passage of the electric fluid may be from the compression of the air, yet the development of electricity by heat, the influence of heat on magnetic bodies, and that of light on the vibration of the compass, show an occult connexion between all these agents, which probably will one day be revealed. In the mean time it opens a noble field of experimental research to philosophers of the present, perhaps of future ages.
As we have noted, Faraday was rather dismissive of the experiment of Morrichini. However, the same year that Somerville was ‘confirming’ it herself, another researcher was demonstrating a light/magnet relationship with an experiment that Faraday did find compelling. Samuel Hunter Christie, the inventor of a technique for measuring an unknown electrical resistance (now inaccurately known as the Wheatstone bridge), also had a great interest in measurements of the Earth’s magnetic field. In an 1823 paper, he measured the strength of the Earth’s magnetic field relative to a pair of bar magnets using a magnetic compass needle. He noted, however, that the magnetic strength of the compass needle could depend upon its temperature, so he set out to examine the behavior of his magnetic needle in different conditions. In Christie’s words ( “On magnetic influence in the solar rays,” Phil. Trans. Roy. Soc. Lond. 116 (1826), 219),
In order to ascertain the effect which changes of temperature have on the times of vibration of a needle, it is necessary to know the temperature of the needle itself during the observations, and I saw no better means of ascertaining this, even approximately, than to vibrate it in the shade and then exposed to the rays of the sun, and to consider the temperature of the needle to be that indicated by a thermometer near to it. On my first doing this, I found, that although I could easily mark the 50th vibration when the needle was shaded, I could not distinguish beyond the 40th when it was exposed. I at the same time found that the time of vibration was slightly diminished at the higher temperature, instead of being increased, as I had reason to expect. As however the needle was not vibrated in the same spot in the two cases, the diminuition in the time of vibration and of the arc when it was exposed, might be independent of the change of temperature and of any influence in the solar rays. To avoid any uncertainty arising from difference of disturbing causes in two situations, I placed the compass out of doors, with a screen composed intirely of wood, supported at the height of four feet above it, and by removing which the rays of the sun struck directly on the needle. A thermometer having the bulb near to the compass-box indicated nearly the temperature of the needle. When the shutter was up, so that the needle vibrated in the shade, I could very distinctly note the 100th vibration; but when it was removed and the needle vibrated exposed to the sun’s rays, I could not so distinctly mark the 75th.
To paraphrase: We are all familiar with how a compass needle swings back and forth when one moves it, eventually settling on magnetic north. Christie noted that this swinging disappears more rapidly (i.e. the motion of the needle is more damped) when in direct sunlight than when in the shade.
I included the additional description of the experiment to illustrate that Christie was no rookie scientist, and not one to make easy mistakes in his setup. Later in the paper (and in its sequel) he notes that a similar damping occurs for nonmagnetic needles of comparable size, but it is of a much lesser extent. He also considers the effects of stray air currents, and is careful to make certain to compare sunlit and shaded conditions only when the overall temperatures are equal.
Faraday added a footnote to his work on Faraday rotation, to give credit to Christie’s work:
I should not have written “for the first time” as above, if I had remembered Mr. Christie’s experiments and papers on the Influence of the Solar Rays on Magnets…
What are we to make of this experiment? Are the results an artifact of some subtle systematic error? Is some other indirect thermal effect at play, or some sort of radiation reaction? Or is it an actual, crude demonstration of light/magnet interactions? The latter seems very unlikely, but there are so many variables in the experiments that it is not easy to isolate what actually is going on.
If anything, the questions raised by these earlier researches highlight the difficulties those early scientists had in dealing with phenomena which were almost entirely unknown. They also highlight the brilliance of Faraday’s experimental discovery of Faraday rotation, which we will discuss in a upcoming post.
To be clear, I’m not arguing that either Morrichini or Christie truly demonstrated a light/magnetism connection. Their work does show that many researchers had suspected such a connection before Faraday and Maxwell.
I am also very interested in understanding what they actually did observe, if possible! Any thoughts?