Electricity Magnetism And Light

[Gene Cryder]

Thisis, 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!

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.

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.

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.

As for the compass needle, that is a bit tougher to explain, we have all seen the black and white suspended metal in the vacuum bulb being turned by light radiation, so there is definately enough physical force in light to have some direct motive force on small light metal, but it seems to me that would not be enough effect to be visibly noticeable on a compass needle. I have never read about this experiment, it is intriguing.

With the publication of "A Dynamical Theory of the Electromagnetic Field" in 1865, Maxwell demonstrated that electric and magnetic fields travel through space as waves moving at the speed of light. He proposed that light is an undulation in the same medium that is the cause of electric and magnetic phenomena.[4] The unification of light and electrical phenomena led to his prediction of the existence of radio waves. Maxwell is also regarded as a founder of the modern field of electrical engineering.[5]

James Clerk Maxwell was born on 13 June 1831[16] at 14 India Street, Edinburgh, to John Clerk Maxwell of Middlebie, an advocate, and Frances Cay,[17][18] daughter of Robert Hodshon Cay and sister of John Cay. (His birthplace now houses a museum operated by the James Clerk Maxwell Foundation.) His father was a man of comfortable means[19] of the Clerk family of Penicuik, holders of the baronetcy of Clerk of Penicuik. His father's brother was the 6th baronet.[20] He had been born "John Clerk", adding "Maxwell" to his own after he inherited (as an infant in 1793) the Middlebie estate, a Maxwell property in Dumfriesshire.[17] James was a first cousin of both the artist Jemima Blackburn[21] (the daughter of his father's sister) and the civil engineer William Dyce Cay (the son of his mother's brother). Cay and Maxwell were close friends and Cay acted as his best man when Maxwell married.[22]

Maxwell's parents met and married when they were well into their thirties;[23] his mother was nearly 40 when he was born. They had had one earlier child, a daughter named Elizabeth, who died in infancy.[24]

When Maxwell was young his family moved to Glenlair, in Kirkcudbrightshire, which his parents had built on the estate which comprised 1,500 acres (610 ha).[25] All indications suggest that Maxwell had maintained an unquenchable curiosity from an early age.[26] By the age of three, everything that moved, shone, or made a noise drew the question: "what's the go o' that?"[27] In a passage added to a letter from his father to his sister-in-law Jane Cay in 1834, his mother described this innate sense of inquisitiveness:

He is a very happy man, and has improved much since the weather got moderate; he has great work with doors, locks, keys, etc., and "show me how it doos" is never out of his mouth. He also investigates the hidden course of streams and bell-wires, the way the water gets from the pond through the wall....[28]

Recognising the boy's potential, Maxwell's mother Frances took responsibility for his early education, which in the Victorian era was largely the job of the woman of the house.[29] At eight he could recite long passages of John Milton and the whole of the 119th psalm (176 verses). Indeed, his knowledge of scripture was already detailed; he could give chapter and verse for almost any quotation from the psalms. His mother was taken ill with abdominal cancer and, after an unsuccessful operation, died in December 1839 when he was eight years old. His education was then overseen by his father and his father's sister-in-law Jane, both of whom played pivotal roles in his life.[29] His formal schooling began unsuccessfully under the guidance of a 16-year-old hired tutor. Little is known about the young man hired to instruct Maxwell, except that he treated the younger boy harshly, chiding him for being slow and wayward.[29] The tutor was dismissed in November 1841. James' father took him to Robert Davidson's demonstration of electric propulsion and magnetic force on 12 February 1842, an experience with profound implications for the boy.[30]

Maxwell was sent to the prestigious Edinburgh Academy.[31] He lodged during term times at the house of his aunt Isabella. During this time his passion for drawing was encouraged by his older cousin Jemima.[32] The 10-year-old Maxwell, having been raised in isolation on his father's countryside estate, did not fit in well at school.[33] The first year had been full, obliging him to join the second year with classmates a year his senior.[33] His mannerisms and Galloway accent struck the other boys as rustic. Having arrived on his first day of school wearing a pair of homemade shoes and a tunic, he earned the unkind nickname of "Daftie".[34] He never seemed to resent the epithet, bearing it without complaint for many years.[35] Social isolation at the Academy ended when he met Lewis Campbell and Peter Guthrie Tait, two boys of a similar age who were to become notable scholars later in life. They remained lifelong friends.[17]

Maxwell was fascinated by geometry at an early age, rediscovering the regular polyhedra before he received any formal instruction.[32] Despite his winning the school's scripture biography prize in his second year, his academic work remained unnoticed[32] until, at the age of 13, he won the school's mathematical medal and first prize for both English and poetry.[36]

Maxwell's interests ranged far beyond the school syllabus and he did not pay particular attention to examination performance.[36] He wrote his first scientific paper at the age of 14. In it he described a mechanical means of drawing mathematical curves with a piece of twine, and the properties of ellipses, Cartesian ovals, and related curves with more than two foci. The work,[17][37] of 1846, "On the description of oval curves and those having a plurality of foci" [38] was presented to the Royal Society of Edinburgh by James Forbes, a professor of natural philosophy at the University of Edinburgh,[17][37] because Maxwell was deemed too young to present the work himself.[39] The work was not entirely original, since Ren Descartes had also examined the properties of such multifocal ellipses in the 17th century, but Maxwell had simplified their construction.[39]

Maxwell left the Academy in 1847 at age 16 and began attending classes at the University of Edinburgh.[40] He had the opportunity to attend the University of Cambridge, but decided, after his first term, to complete the full course of his undergraduate studies at Edinburgh. The academic staff of the university included some highly regarded names; his first year tutors included Sir William Hamilton, who lectured him on logic and metaphysics, Philip Kelland on mathematics, and James Forbes on natural philosophy.[17] He did not find his classes demanding,[41] and was therefore able to immerse himself in private study during free time at the university and particularly when back home at Glenlair.[42] There he would experiment with improvised chemical, electric, and magnetic apparatus; however, his chief concerns regarded the properties of polarised light.[43] He constructed shaped blocks of gelatine, subjected them to various stresses, and with a pair of polarising prisms given to him by William Nicol, viewed the coloured fringes that had developed within the jelly.[44] Through this practice he discovered photoelasticity, which is a means of determining the stress distribution within physical structures.[45]

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