Imágenes de páginas
PDF
EPUB

interrogates Nature, is no longer listened tains an account of their verification by M. to with incredulity, nor pointed at with Pouillet, a French philosopher, some of scorn. If, indeed, any complaint can be whose remarks we shall have occasion to made against the present tendency of pub- quote. To the third work, that of Reichlic opinion in this matter, it is that the cur- enbach, we purpose to reser, both because rent has set in an entirely opposite direc-it bears somewhat upon our present subtion,-—it is that the reaction from the ject, and for the reason that it has attracted indifference and obstinacy of past ayes but little attention in this country. carries us to the other extreme, and leads In the present article, then, it is our into the formation of great anticipations tention to give such a sketch of the leading from trifling, insignificant, and insufficient features of the discoveries recorded in the data. But, comparatively speaking, this is volumes before us, as we trust will be inof little importance-it is an error on the telligible ; for which purpose, however, as right side; Time, the great leveller, will well as that the reader may the more corsoon separate the grain from the husk; rectly judge of their novelty and importdiscoveries of real importance will remain ance, it will be necessary briefly to sketch as permanent additions to our knowledge, the history of this branch of science. while ill-founded anticipations and theories The attractive power exerted by the will inevitably be buried in oblivion, or loadstone over iron, appears to have been only be remembered as examples of human known in times of very remote antiquity, fallibility, "to point a moral, or adorn a It is mentioned by Homer, Pythagoras, tale."

Aristotle, Euripides, and Pliny. The latMagnetism has, equally with other deter author, indeed, seems further to have partments of physical science, been distin- been acquainted with the property of inguished for this rapid onward progress. duction, or the power possessed by the Scarcely a quarter of a century back, all loadstone of communicating its virtue to magnetic instrumente, with the exception iron placed in its immediate vicinity; for of the mariner's compass, were but philo- he mentions the fact that an iron ring, supsophic toys. Since that period, however, ported by a loadstone, will in its turn susthe correlation of the two forces, mugnet- tain the weight of another. But although it ism and electricity, has not only been thus appears clear that these two phenomeclearly proved, but has likewise been taken na were known, yet its directive power or advantage of in the construction of an in- polarity, that is to say, its property of strument, certainly one of the wonders of pointing north and south, seems to belong the age, by which time and space are al- to a later date. True, it is generally asmost annihilated—we allude to the electric serted that the Chinese were acquainted telegraph; and, more recently still, the with, and took advantage of, this directive persevering researches of our illustrious power from a very early period. In a work countryman, Dr. Faraday, have led to the entitled, 'General History of China,' by P. discovery of the intimate connexion exist- Dubalde, the following passage occurs. ing between this force and another of the Speaking of some ambassadors, the author imponderables-light; and shown to us, says :moreover, the real nature of the action exercised by magnetism over all matter,-a

“ After they had their audience of leave in problem whose solution has been in vain order to return to their own country, Tcheonattempted at different periods by the most side pointed 10 the north, and the opposite side

Kong gave

them an instrument, which on one distinguished philosophers. The new fields to the south, to direct them better on their way of science thus opened to us, promise an home than they had been directed in coming ample harvest of discoveries-discoveries to China. The instrument was called Tchithe more likely to follow, from the eager- Nan, which is the same as the Chinese now ness with which the necessarily brief an- give to the sea-compass; and this has given nouncements in some of our public jour- occasion to think that Tcheon-Kong was the nals have been every where received, and the 22d cycle, more than 1010 years before

inventor of the compass. This happened in the remarkable celerity with which the ex- the Christian era.” periments have been tested and verified in all parts of the Continent. The experi- In further support of this assertion, Dr. ments and deductions of Dr. Faraday are Gilbert affirms that Paulus Venetus brought detailed in the volume first in the list the compass to Italy from China, in the year which heads this article; the second con- 1260. But, unfortunately for this assertion, it is clear from many authors that the com- hazarded by different authors, but it was pass was in use in Europe in the twelfth not until the year 1759 that a rational hycentury. Cardinal James de Vitri, who pothesis was devised by Epinus, which emflourished about the year 1200, mentions braced and explained almost all the phenomthe magnetic needle in his 'History of ena observed by previous authors. He conJerusalem,' and he adds, that it was of in-ceived that in all magnetic bodies there exdispensable utility to those who travelled isted a power which may be called the mag. by sea. In an old French poem, entitled netic Auid, whose particles repel each other La Bible Guiot, still extant in the Royal with a force inversely as the squares of the Library at Paris, allusion is evidently made distance; that the particles of this fuid atto the magnetic needle. Its author was tract the particles of iron, and are atGuiot de Provence, who lived at the latter tracted by them in return with a similar part of the twelfth century. The passage force; that the particles of iron repel each is so remarkable, that we are tempted to other according to the same law; that the subjoin a translation :

magnetic fuid moves through pores of iron " This (the pole) star does not move, and and soft steel with very little obstruction; they (the mariners) have an art which can- but its motion is more and more obstructnot fail by virtue of the magnet-an ugly, ed as the steel increases in hardness or tembrownish stone, to which iron adheres of per, and it moves with the greatest difficulty its own accord. They look to the right in hard-tempered steel or the ores of jron. point, and when they have touched a In this state did the science of magnetneedle, and fixed it on a bit of straw ism remain for another half century. No lengthwise, exactly in the middle, the new facts were added until the year 1800, straw keeping it up, the point turns when the celebrated electrician Coulomb straight and unerringly towards the star. directed his powerful mind to the subject. When the night is so dark and gloomy that Provided with the delicate instrument, the you can neither see star nor moon, they torsion balance, he determined the correct bring a light to the needle: may they not law of magnetic attraction and repulsion; then assure themselves of the situation of he showed that the magnetism in the midthe star by the direction of the point ? dle of a bar was imperceptible, and that it Thus, the mariner is enabled to keep the increased according to a regular law, and proper course. It is an art which cannot with extreme rapidity towards each of the deceive."

poles. He established the important fact, We think there can be no question, from that the magnetic power resides on the surthe whole of this singular passage, that the face of iron bodies and is entirely indecompass is clearly referred to.

pendent of their mass. He also directed That ferruginous substances always pos- his attention to the effect of temperature sess a greater or less degree of magnetism, on magnets; he found that the magnetism has long been known. One Julius Cæsar, of a bar magnetized to saturation diminisha surgeon of Rimini, first observed the con- ed greatly by raising its temperature from version of iron into a magnet. In 1590, 12 to 680 degs. Reaumur, and that when a he noticed this effect on a bar of iron, magnetic bar was tempered at 780, 860, which had supported a piece of brickwork and 950 degs. Reaumur, the development on the top of a tower of the church of St. of its magnetism was gradually increased, Augustin. The very same fact was ob- being more than double at 900 degs. of what served about 1630 by Gassendi, on the it was at 780 degs. Up to the period of these cross of the church of St. John, at Aix, researches of Coulomb, iron, steel, nickel, which had fallen down in consequence of and cobalt were regarded as the only maghaving been struck by lightning. He netic bodies; but in the year 1802 he anfound the foot of it wasted with rust, and nounced to the Institute of France, that all possessing all the properties of a loadstone. bodies whatever are subject to the magnetic

During the succeeding century, the at- influence, even to such a degree as to be tention of those philosophers who devoted capable of accurate measurement. Since themselves to the study of the phenomena this announcement of Coulomb, the belief of magnetism was exclusively confined to that magnetism affected all matter in the the directive power of the needle, its varia- same manner as iron, although in a less detion, the variation of the variation, and the gree,-a belief, as we shall subsequently dip. Various speculations respecting the find,utterly erroneous,-has almost univercause of the phenomena of magnetism were sally prevailed among philosophers.

Such, then, was the state of magnetic which we shall immediately have to descience up to the year 1820. The analo- scribe. gies between the phenomena of magnetism This discovery of Ersted, where elecand those of electricity, in their general tricity was made to evolve magnetism, incharacter, in the laws which govern them, duced philosophers to attempt to produce a and in the various combinations they pre- converse effect; that is, to educe electricity sent, are so extensive and so remarkable, from a permanent magnet. During the ten as naturally to lead to the belief that the years succeeding the publication of Erforces themselves must be closely allied sted's researches, unnumbered experiments to each other. This connexion between were made to produce this effect; but all magnetism and electricity was a favorite these experiments failed, for the reason that subject of speculation and inquiry among all their devisers were led away with the philosophers during the last, and the com- expectation of making a stationary magnet mencement of the present century. Many a source of electricity; in which, had ihey attempts were made to solve this seductive succeeded, they would have realized a reproblem, which continued, however, to baf- sult now deemed so absurd and hopeless, fle the labors of each succeeding experi -perpetual motion. The error of such mentalist, who multiplied his efforts and anticipations was seen by Faraday; and in varied his processes withont approaching the year 1831 he proved that to render any nearer to the point he aimed at; and magnetism a source of electricity it was also to elude the reasonings of those who necessary to superadd to the former, motheorized upon every new fact until they tion : that when a piece of metal is passed bewildered both themselves and their read- before a single pole, or between the opposite ers in the mazes of visionary and con- poles of a magnet, electrical currents, transflicting hypotheses. To Ersted was re- verse to the direction of motion, are proserved the honor of discovering the link duced across it; and that magnets, while which binds these two sciences. In the in motion, induce electricity in contiguous year 1820, Ersted proved that the two conductors, the direction of the electric forces, electricity and magnetism, act upon currents being tangential to the polar direceach other, not in straight lines, but at tion of the magnet. From these fundamenright angles to each other; that is to say, tal laws originates the science of magnetothat bodies which conduct a current of electricity, the true converse of electroelectricity tend to place magnets at right magnetism. By this science are we placed angles to them, and inversely, magnets have in possession of one of the most beautiful a tendency to place such conducting bod- and instructive instruments of modern scijes at right angles to them; and this ten- ence, the magneto-electric machine, by the dency is, of course, in proportion to the which we see exemplified the close connexpower of the electric current. An electric ion between, if not the identity of, the eleccurrent, therefore, appears to have a mag-tric and magnetic forces; the same heating, netic action, in a direction cutting its own magnetizing, and decomposing power, the at right angles; or, supposing its section to same velocity of motion, the same physiobe a circle, tangential to it; if, then, we logical and chemical effects, are thus shown reverse the position, and make the electric to be common to both :current form a series of tangents to an imaginary cylinder, this cylinder should be a "In the discoveries of Ersted and Faraday," magnet. This is effected in practice by observes an eloquent author of the present day, coiling a wire as a helix or spiral; and this,

we having a striking example of the superiwhen electrified, is, to all intents and pur- and didactic theories ; in each case a connex

ority of intuitive perception over formal rules poses, a magnet. A soft iron core placed ion was generally suspected and strongly bewithin such a helix has the property of lieved; theories without number were proconcentrating its power; and then, by con- pounded, and fallacies authoritatively enforced; nexion or disconnexion with a voltaic bat- failure succeeded failure, until the eagle-flight tery, we can at pleasure make or unmake of genius swooped upon the citadel, which a powerful magnet. The magnets former-theory had vainly attempted to scale." ly in use are utterly insignificant when Such then, in brief, is the history of the compared with these electro-magnets; and science of magnetism; and we now turn it is only by the vastly increased power so 10 the consideration of the researches of obtained that Professor Faraday has been Faraday. The first paper in the Philoenabled to make the important discoveries sophical Transactions before us is, “On

[ocr errors]

the magnetization of Light, and the illumi- “The violet light was obtained in the usual nation of magnetic lines of Force;" and manner by means of a common prism, and to this we shall necessarily first direct at- was collected into a focus by a lens of suffitention.

cient size. The needle was made of soft wire, In common with many philosophers, Dr. polarity, nor any power of auracting iron

and was found, upon trial, to possess neither Faraday has long entertained an opinion, | flings. It was fixed horizontally in a support that the various physical forces have one by means of wax, and

such a dir on as common origin, or, in other words, are so to cut the magnetic meridian at right angles. directly related, and mutually dependent, The focus of violet rays carried slowly along that they are convertible, as it were, one the needle, proceeding from the centre, tointo another, and possess equivalents of wards one of the extremities, care being taken

never to go back in the same direction, and power in their action. We have said that

never to touch the other half of the needle. this opinion was not confined to Dr. Fara- At the end of half an hour after the needle day alone. In the season 1843–44, Mr. had been exposed to the action of the violet Grove, the Professor of Natural Philosophy rays, it was carefully examined, and it hall acin the London Institution, delivered in that quired neither polarity nor any force of atinstitution a course of lectures on the traction; but after continuing the operation “ Correlation of Physical Forces.”

His twenty-five minutes longer, when it was taken

off and placed on its pivot, it traversed with object was to show that motion, chemical great alacrity, and settled in the direction of affinity, heat, light, electricity, and magnet- the magnetic meridian, with the end, over ism, are all convertible affections of mat- which the rays had passed, turned towards ter—that either being taken as an initial the north. It also attracted and suspended a mode of force is able to produce any of fringe of iron filings. The extremity of a the others; thus moving bodies may be needle that was exposed to the action of the made, mediately, or immediately, to pro- violet rays repelled the north pole of a com

This effect was so distinctly duce heat, light, electricity, chemical af marked, as to leave no doubt in the minds of finity, or magnetism. Matter affected by any one present, that the needle had received its chemical affinity may be made to produce magnetism from the action of the violet rays."* motion, heat, light, electricity, and magnetism, and so of the rest. In each lecture, In the summer of 1825, Mrs. Somerville one of the above forces was taken as the was induced, by the clearness of the weathinitial or starting point, and it was shown er, to make some experiments on the subexperimentally how the others were project

. A sewing-needle, an inch long, and duced by it.

devoid of magnetism, was half covered with Of all these physical forces, however, paper, while the other half was exposed to light had most completely resisted the ef- the violet rays of the spectrum, five feet disforts of philosophers to demonstrate, expe- tant from the prism. In two hours it acrimentally, its connexion with the other quired magnetisin, the exposed end exhibitforms under which the forces of matter are ing north polarity. The indigo rays promade manifest. That a relation did exist duced an equal effect, and the blue and the between them was not doubted, and some green the same in a less degree. In no one experiments, first devised by Dr. Morichini, instance was magnetism produced by the an eminent physician at Rome, gave great- yellow, orange, or red rays; though, in er force to this opinion. He announced some instances, the same needles were exit as an experimental fact that an unmagnet- posed to their influence for three successive ized needle, exposed in a particular man- days; neither did the calorific rays of the ner to the concentrated violet rays of the spectrum produce any sensible effect. solar spectrum, became magnetic. These Pieces of clock and watch-spring were also experiments were successfully repeated by tried with similar success, and were found Dr. Carpi, at Rome, and the Marquis Rí- to be even more susceptible of this peculiar doli, at Florence; but the failure of the magnetic influence than needles. The vioexperiments, when tried by others, created let rays concentrated by a lens produced great doubt of the accuracy of the result magnetism in a shorter time than ihe prism as reported by Morichini. Sir Humphrey alone. But notwithstanding these simple Davy and Professor Playfair both witnessed and well-conducted experiments, which and paid diligent attention to the experiments of Morichini, the results of which

* It is a fact, which may perhaps be unknown were thus subsequently described by the most effective in the decomposition of the vari seemed to set the question at rest from the then made to pass through the length of a distinct and decided character of the re- square piece of heavy glass composed of sisults, and notwithstanding the confirmation licated borate of lead about two inches of them, to a certain degree, by Mr. Chris- long, and five-tenths of an inch thick, on tie, by a different mode of observation, a its emergence from which it passes through general opinion seems ever since to have a Nicholls' eye-piece, revolving on a horiprevailed, that light does not exercise any zontal axis, so as to intercept the ray, or decided effect in producing magnetism. allow it to be transmitted alternately in the

to some readers, that the violet rays are also the latter philosopher :

ous compounds used as photographic agents.

It was in this state of the subject, that different phases of its revolution. The Dr. Faraday was induced to turn his atten- heavy glass or diamagnetic, is placed either tion to it, to endeavor to discover the direct between the two poles of a powerful horserelation of light and electricity, and their shoe electro-magnet, or the contrary poles mutual action in bodies subject jointly to of two cylinder magnets, so arranged as their power. For many years the experi- that the line of magnetic force resulting ments carried on with this view, not only from their combined action coincides with, by himself, but by other philosophers, were or differs but little from, the course of the unattended with any definite result; and it ray in its passage through the glass. If in was not until the latter part of last year that this state of circumstances, the Nicholls' his long.continued and persevering efforts eye-piece being so turned as to render the received their reward; he then succeeded polarized ray invisible to the observer lookin magnetizing and electrifying a ray of ing through it, the force of the electro-mage light, and in illuminating a magnetic line net be developed by sending an electric of force.

current through its coils, the image of the But before we proceed to describe the lamp-flame immediately becomes visible, means by which this important and inter- and continues so as long as the arrangeesting result was effected, it will be neces- ment continues magnetic.

On stopping sary to define the meaning of certain terms the electric current, and so causing the which are continually used, and this we magnetic force to cease, the light instantly cannot do better than in Dr. Faraday's own disappears; these phenomena may be rewords :

newed at pleasure, at any instant of time,

and upon any occasion, showing a perfect “ By line of magnetic force, or magnetic dependence of cause and effect. The same exercise of magnetic force which is exerted in phenomena may be produced by the action

of the lines usually called magnetic curves, and

a good ordinary steel horse-shoe magnet, which equally exist as passing to or from mag- no electric current being used. The renetic poles, or forming concentric circles round sults are more feeble, but still sufficient to an electric current. By line of electric force, show the perfect identity of action between I mean the force exerted in the lines joining electro-magnets and common magnets in two bodies, acting on each other according to this their power over light. At the same the principles of static electric induction, which time an observer, to observe the phenome may also be either in curved or straight lines. By a diamagnetic, I mean a body through non, should commence by using a powerful which lines of magnetic force are passing, and magnet, as it is probable that at first he which does not, by their action, assume the would not be able to detect it with a weak usual magnetic state of iron or loadstone." magnet. The electro-magnet originally

used by the discoverer was of such power The fundamental experiment which es- that the poles would singly sustain a weight tablishes the link of connexion between two of from twenty-eight to fifty-six or more great departments of nature is as follows: a pounds. The best form of battery to be ray of light issuing from an Argand lamp is employed is the excellent one of Mr. Grove's polarized in a horizontal or any other plane construction, of which from five to ten pairs by reflection from a surface of glass;* it is of plates will be ample.

The force thus impressed, by the mag* A modification of this apparatus, excellently netic action, upon the diamagnetic, is that adapted for the exhibition of this experiment was a short time since exhibited at the London Insti- of rotation ; for when the image of the lamptution. It consists simply of a glass tube, furnished at either extremity with a tourmaline, and placed piece first used by Dr. Faraday, is made to revolve within a core of soft iron which is wound 'round so as to intercept or transmit the polarized ray. with covered copper wire. The ray of light is When the ray is invisible, connexion between a polarized by the first tourmaline-the second battery and the heli wire immediately restores tournaline, which answers to the Nicholls' eye. ( it to view.

« AnteriorContinuar »