CHAPTER 2
MAGNETIC PHENOMENA
It was long ago observed that if glass is rubbed by silk, or a piece of sealing-wax or hard rubber by fur or wool, an effect occurs similar to that noted by Thales when amber is rubbed by similar materials—i. e., light bodies such as bits of dry paper, pith, etc., will cling to the surface of the substance. After coming in contact with the attracting substance, the bits of paper, straw, etc., are then repelled.
If a ball made of pith be suspended at the end of a silk thread, and a glass rod which has just been rubbed with silk be brought close to the ball, the pith-ball immediately flies to the rod, clinging to it for a time. Then it jumps away, and instead of hanging vertically, seems to be pushed away from the glass by a mysterious force. A second ball, treated like the first, and brought near the first, is violently repelled. But if one ball is charged from the glass and one from the wax, they attract instead of repelling each other. Two pieces of glass or two pieces of wax repel each other.
A similar attraction and repulsion was early observed between the poles of the magnet. This influence seems to be transmitted by some invisible agency or medium across the intervening space between the bodies, and in this respect the force does not differ from that acting between the moon and the earth, or the earth and the sun. And just so, if a light piece of iron is placed near a magnet, it moves to the magnet and clings to it; but if the magnet is the lighter of the two bodies, it moves toward the piece of iron.
Although Thales had attempted to explain the cause or nature of magnetic attraction as long ago as the end of the seventh century B. C., or in the first quarter of the sixth century (about 2,500 years ago), it was not until the year 1582 A. D. that Dr. William Gilbert (1540-1603), of Colchester, physician to Queen Elizabeth, made the first experimental study of magnetic phenomena. It is to Dr. Gilbert that we owe the name electricity as applied to this force, derived from his vis electrica.
By 1600, Dr. Gilbert had published his epochal work “De Magnete”, which not only contained the first rational treatment of magnetic and electrical phenomena, but was also virtually the first scientific work published in England. It is to this truly great treatise that must be traced the beginnings of the science of electricity.[4]
Throwing aside, as useless, mere philosophical speculation as to the nature of magnets, Gilbert explained in his book how practical experiments should be carried out. He insisted that it is to nature herself that we must apply for the answers to problems in “natural history”. Gilbert’s particular objective was not, however, discovery of the laws of magnetism or electricity; what he most desired to learn was the composition of the earth: he wished to know through actual research just what is its innermost constitution. His experiments led him to the conclusion that the earth is a magnet. It may, indeed, be considered a huge spheroidal lodestone.
Gilbert told his readers to take a piece of lodestone (natural magnetic iron) of convenient size, turn it on a lathe to the form of a ball, then place on the terella (as he called the spherical lodestone) a piece of iron wire. It will then be observed that the ends of the wire “move round its middle point.”[5]
Lodestones, fragments of magnetite (Fe3O4), are said to have been first discovered at Magnesia, in Asia Minor,—hence the word magnetism. Some of the earliest references to the lodestone relate to its property of lying in a north-and-south direction when an elongate stone is freely suspended, one particular end always pointing northward, just as the great magnet the earth, or the mariner’s compass-needle, has two opposite magnetic poles. The location of the poles of a disk-shaped stone is readily found by turning it round in the presence of a compass-needle.[6]
Iron and steel are more strongly magnetic than any other metals. While only one kind of iron ore is naturally magnetic—forming magnets—the property of magnetism may always be given to any kind of iron or steel. One need only strike an iron bar while it is lying in a north-south position, or rub the iron with a magnet, and it becomes a magnet. If it is desired to make a permanent magnet, steel must be employed. A compass-needle is therefore made of magnetized steel. If balanced upon a pivot, the positive pole of the needle will point (roughly) towards the earth’s north geographical pole.[7]
A compass-needle is also a “dipping needle”, unless the suspended magnetized needle lies about half way between the earth’s magnetic poles. The north magnetic pole lies below the earth’s surface—at an unknown depth—at the extreme northeastern corner of the continent of North America; and the corresponding south magnetic pole on the edge of the Antarctic continent—King George’s Land—about 2,300 miles south of Australia. These magnetic poles do not correspond even roughly with the geographic poles, nor does the magnetic equator by any means correspond with the geographic equator.
Only a small section of the magnetic equator runs north of the true (geographic) equator—e. g., from the coast of Brazil to the coast of Kamerun (Africa).
According to Prof. T. J. J. See, “the whole magnetic system has been pushed southward 200 miles by bodily displacement of both poles towards the ocean hemisphere.” This eminent physicist-astronomer also stated (in 1922) that his researches led him to the discovery that the two magnetic poles are at unequal depths in the earth, the North Pole being much deeper than the South Pole, “with the result that the total magnetic forces in the southern hemisphere are considerably stronger than in the northern hemisphere.”[8]
It was long ago discovered that if one starts northward from the magnetic equator, the compass-needle soon begins to dip downward (and northward). At the southern border of the United States, the downward inclination amounts to about 57 degrees. At the borders of North Dakota and Maine the dip is about 76 degrees. By the time Hudson Bay is reached the needle assumes a vertical position. This means that it is here suspended immediately over the north magnetic pole itself. At the magnetic equator in Peru, a needle suspended by a thread is exactly balanced. Dr. See states that at the North and South Poles there is a downward pull—by the magnetic force—of just one millionth of the gravitational force, while in Peru the total magnetic force is precisely one ten millionths of gravitation.
It has been found that both the North and the South Poles are anything but fixed in position. They “wander about in their subterranean region”. In the course of centuries, the compass-needle swings from west of north, and then to the east. Even the amount of the dip slowly changes, in a periodic way, and at every point on the earth. For example, in 1576, the north end of the needle at London dipped at an angle of 71 degrees 50 minutes. By 1720 the angle had increased to 74 degrees 42 minutes—almost up and down. Since then, the dip at London has continually decreased. At the present time we are puzzled by the fact that the inclination of the dip is 66½ degrees at London and more than 70 degrees at Washington.
It has long been known that variations in magnetic declination of the delicately mounted needles, in observatories, are directly correlated with solar disturbances. The late Dr. A. Wolfer (sometime director of the Zurich Observatory) was the first to show us how closely the curve of the sun-spot activity rises and falls with the fluctuations of magnetic declinations.
Before attempting to explain the peculiarities of magnetic action in terms of the modern electromagnetic theory, it will be well to recall certain stages of progress in the development of this theory. This plan will permit elucidation of the theory itself by “easy steps”.
FOOTNOTES:
[3] Cf. Whittaker, E. T., “A History of the Theories of the Ether and Electricity from the Age of Descartes to the Close of the Nineteenth Century,” Dublin and London, 1910. See also, Comstock and Troland, “The Nature of Matter and Electricity,” New York, 1917; Steinmetz, Dr. Charles P., “Elementary Lectures on Electric Discharges, Waves and Impulses and Other Transients,” New York, 1914; and Starling, Dr. Sydney G., “Electricity,” London and New York, 1922.
[4] On the Continent, experimental work in other fields was already in progress, thanks to the genius of Descartes, Galileo and other founders of modern science. Gilbert, like Harvey, spent some years in Italy, coming under the direct influence of the great Italian physicist-astronomer-physician Galileo. Harvey was in Padua (1598-1602) during Galileo’s professoriate. The introduction of scientific methods in England at this time may well be credited to Italian and French influences.
[5] Gilbert’s book is usually referred to simply as “The Magnet,” but the full title is: “Concerning the Magnet and Magnetic Bodies, and Concerning the Great Magnet the Earth: A New Natural History (Physiologia) Demonstrated by Many Arguments and Experiments.”
[6] Magnetite does not always possess polarity. It is called “lodestone” only when it does. It occurs not only in the form of more or less massive stones, but also as loose sand and in earthy forms.
[7] The fact that a lodestone possesses two “poles” was discovered in the thirteenth century by Petrus Peregrinus, of Picardy, while he was experimenting with a spherical lodestone and a needle.
[8] From notes taken at a lecture by Dr. See before the California Academy of Sciences in 1922. Dr. See, in charge of the United States Naval Observatory at Mare Island (California), presented in the lecture “A New Theory of the Ether,” in which he outlined the grounds upon which he based his new theory of a direct connection between magnetism and universal gravitation. It is highly interesting, in this connection, to learn that Dr. Albert Einstein, in collaboration with Professor Eddington (of Cambridge)—working on the principle of Relativity—has discovered a connection between the earth’s power of attraction (gravitation) and electricity.