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Memoir on Induction. By M. M. Masson and BREGUET.
(Extracted by the Authors.)* When a very long wire is traversed by a voltaic current, the points situated at equal distances from the extremities of this wire, appear to us to be charged with static electricity of contrary signs, capable of charging a condensing electroscope.
At the moment of closing and breaking the current, these tensions seem to augment and to acquire a great vigour by twisting the wires into helices.
When the wire is wound into an helix, the tension is so much augmented at the point of interruption of the circuit, that we may easily obtain sparks of two centimetres long in a vacuum.
These phenomena of induction appear to be due to actions exercised at a distance by the static electricities on the neighbouring wires, and are also presented in the phenomena of electric influence obtained by the electrical machine.
The electric light obtained in vacuo by the extra currents, or the currents of induction, present the same character as that which is obtained in the same circumstances with electrical machines or the Leyden jar.
When two helices are placed one on the other, the one receiving the current of the pile, we experience shocks on taking hold of one extremity of the extra current, and one extremity of the primary helix ; if another person takes the two other extremities which were left at liberty, the shocks will be increased.
Three helices being placed one on the other, if the extremities of the middle one are joined, the interrupted current of the pile passing into the first, will not induce one in the third ; but if we make the ends of the middle helix communicate with a very long wire, it then no longer forms a screen, and the shocks are felt in the third.
When by the disposition and the length of a wire helically wound, we obtain by the extra current, or the current of induction, the electric spark in a vacuum, this light will disappear immediately on putting a cylinder of soft iron into the helix, and will re-appear on withdrawing it.
The static and dynamic states of electricity, are two modes which are susceptible of being transformed one into the other, and by the words intensity and quantity, we must understand equal quantities of active electric forces, which only differ as to the duration of their action.
Thoughts on Magnetism, by RICHARD KIRWAN, Esq., L.L.D.,
F.R.S., and M.R.I.A. 1. There are two ways of explaining a natural phenomenon ; the first, is by discovering the conditions and circumstances of its pro
Comptes Rendus, 23rd August, 1841.
duction and the laws by which its action is governed ; the second, is by shewing its analogy, similarity, or coincidence with some general fact with whose laws or existence we are already acquainted; this last mode is by far the most perfect and satisfactory. In the first sense of the word, electricity and magnetism have been in some measure explained, but in the last sense neither; the primary cause of magnetism in particular has hitherto been supposed to relate to iron alone, or its ores, and to stand unconnected with all other natural phenomena.
2. If therefore any other general fact or power can be discovered to which it bears some analogy or similarity, it may so far be said to be explained. Now such fact or power I think may be assigned, namely, the power of crystalization.
3. By crystalization I understand that power by which the integrant particles of any solid possessing sufficient liberty of motion unite to each other, not indiscriminately and confusedly, but according to a peculiar uniform arrangement, so as to exhibit in its last and most perfect stage regular and determinate forms.
4. This power is now known to be possessed by all solid mineral substances.
5. The forms which crystals, even of homogeneous substances, exhibit, are often very numerous ; however in most cases they may be reduced to a few primordial forms, which, as Abbé Hauy has lately experimentally proved, are derived from certain original forms appertaining to the minutest particles of their concretion.
6. The assemblage of these ultimate particles into visible aggregates, similarly arranged, necessarily requires that one of their surfaces should be attractive of that particular surface of the other, which presents a corresponding angle, and repulsive of that which presents a different angle, otherwise the various regular rhomboidal and other polygon prisms and pyramids, which crystals present to us, could never exist; consequently the minutest prism, being once formed, could never be prolonged if one end of such prisms were not attractive, and the other repulsive of the same given surface.
7. Hence it has been observed that crystalization never takes place in the middle of any solution, but always begins at the surface or on the bottom or sides of the vessels that contain it, for the particles in the middle of the solution being confusedly mixed with each other, and exerting their repulsive as freely as their attractive powers, the one constantly counteracting the other, no sensible accretion of a regular kind could take place, whereas the repulsive power of the uppermost particles, or of those that rest on the sides or bottom of the vessel, is restrained and impeded.
8. The repulsive power of crystallizing substances also appears in many other instances (of the attractive no doubt has ever been formed). Thus if saturate solutions of nitre, common salt, and tartar vitriolate be mixed and set to crystallize, each will crystallize a part, which could not happen if the particles of each of these salts
did not only attract their similar homogeneous, but also repel those of a different species, otherwise the mere casual circumstance of greater proximity to one than to the other would impel them to unite indiscriminately. Again, if a saturate solution of allum be mixed with a turbid mixture of clay, and abandoned to insensible evaporation, after some time the clay will subside and form a dry mass, but in the interior of this mass large regular crystals of allum will be found, whose component particles must, to reunite, have displaced and repelled the particles of clay with which they were surrounded.
9. If to a saturate solution of a salt that difficultly crystalizes, a a crystal of a salt of the same species be inserted, the whole solution will soon be brought to crystalize, as the crystal inserted attracts the particles dissolved, by its different surfaces ; but if a salt of a different nature be inserted this will not happen, crystalization will not be promoted.
10. If to a solution of 2 parts nitre, and 3 parts Glauber's salt in 5 parts water, a crystal of nitre be inserted, the nitre alone will crystallize ; or if instead of nitre a crystal of Glauber be inserted into it, the Glauber alone will crystalize. Do not these experiments fully evince both the attractive and repulsive powers, not only of different salts, but of different surfaces of the same salt?
11. These powers within their proper sphere of action have been found indefinitely great; thus water confined in cannon several inches thick, and exposed to a degree of cold much beneath the freezing point, has been observed to crystalize into ice that burst the metallic impediment opposed to the form it then assumes.
12. The vast difference however attending the development of these two powers (of magnetism and crystalization) will undoubtedly strike many as an insuperable objection to their indentity, yet their direction in all its varieties being exactly the same, difference in other circumstances seems to me to indicate rather a variety of degrees in the same power, than any essential difference in the powers themselves.
I now come to the application of the above principles to the magnetic phenomena. These may in general be reduced to the following, viz. Attraction, Repulsion, Polarity.
1st, Attraction, Repulsion, Polarity. The quantity of iron found on and within such parts of the surface of the globe as we are acquainted with, far surpasses that of any other mineral substance singly taken, or even of many of them taken together ; scarce any stone or metallic ore, or earth, is found free
from it; it enters into their composition in the proportion of from two to eighteen or twenty per cent., and perhaps at a medium we may state it in all of them at six per cent. ; moreover its own ores are of all others, the most common and the most copious ; in many places, particularly in the most northern climates, whole mountains of it are found, and many of them magnetic. When to this consideration we add that of the specific gravity of the globe, which has been found to be 4.5 times heavier than water, notwithstanding the immense quantity of water that covers the greater part of its surface to considerable unknown depths, and notwithstanding that the specific gravity of by far the greater part of the stones and earths it contains, does not exceed, and scarcely amounts even to three times the weight of an equal bulk of water, and that the quantity of mineral substances whose specific weight exceeds four times that of water, is almost infinitely small in comparison to the other known component parts of the globe, and finally, that the weight of most iron ores, is about four or five times that of water; all this I say considered, it is difficult to avoid concluding that the interior part of the globe consists chiefly of iron ore, disposed in one or more aggregate masses ; a conclusion that is farther confirmed, on reflecting that volcanic lavas ejected from the deepest recesses with which we are acquainted, contain from fifteen to twenty or twenty-five per cent. of iron in the state most favourable to magnetic attraction.
Taking then this assertion to be as fully proved as its subject matter is capable of being ascertained, we may deduce from it the following corollaries :
1st. That as the ferruginous matter in the globe being by far the most copious, its universal attractive power is principally seated in the ferruginous part.
2nd. That as all terraqueous matter was originally in a soft state, its parts were at liberty to arrange themselves according to the laws of their mutual attraction, and in fact did coalesce and crystallize in the direction in which they were least impeded by the rotatory motion of the globe, namely, in that which extends from north to south, and principally and most perfectly in the parts least agitated by that motion, namely, those next the centre.
3rd. That this crystallization, like that of salts, might have taken place in one or more separate shoots, or as we may here call them, immense separate masses, each having its poles distinct from those of the other, those in the same direction repulsive of and distant from each other.
In consequence then of the universal law of attraction of the particles of matter to each other, these internal magnets exert a double power of attraction ; the first and most general, on the particles of all bodies indiscriminately in proportion to their density, and the direct or inverse ratio of the squares of their distances, according as those bodies are found within or without the earth's surface; and the second, on bodies of their own species in proportion to their
homogenity, and to the correspondence of the arrangement of their integrant particles with that of the integrant particles of these internal magnets.
A magnet, therefore, is a mass of iron, or of iron ore, whose oxygenation does not exceed 20 per cent., or thereabouts, whose particles are arranged in a direction similar to that of the great internal central magnets of the globe. This I call the magnetic arrangement.
The particles of iron attract each other more forcibly than those of
any other known substance. This appears by its cohesion, hardness, elasticity, and infusibility, in each of which properties, or at least in the combination of most of them, it exceeds all other known bodies.
Hence, a magnet attracts iron when within the sphere of its action, by forcing, in virtue of its attractive power, a certain proportion of its integrant particles into a disposition and arrangement similar to that of its own. For in this case it exerts a double attractive power, that of the particles of iron to each other, which we have seen to be the greatest of all others, and that of crystallizing bodies, which we have also seen to be indefinitely great.
The crystallizing power being at once attractive and repulsive, according to the direction of the surfaces (No. 6), hence we see that one part or end of the magnet must repel that which the other has attracted, as long as the same disposition of parts remains.
The disposition of parts in a particular magnet, being similar to that which obtains in the great internal general magnet, extends in the direction of from north to south. Hence magnets, when at liberty to move with a certain degree of freedom, and iron, when a sufficient number of its particles are arranged in that direction, and has sufficient liberty to conform to it, points to those poles. Hence this property is called Polarity.
The magnetic power is greater or lesser according to the number and homogenity of the particles similarly and magnetically arranged. Hence small magnets may be more powerful than a larger, and hence a magnet will attract a magnetized needle at a greater distance than one not magnetized.
The magnetic power decreases in a certain ratio of the distance of the particles that exercise it. Hence it is strongest at the point of contact, and at the poles, as it is there most unsaturated, and weakest in the central part, which separates the two opposite poles.
When a magnet is broken into small pieces its power is nearly destroyed, because, though the poles should be all of the same kind, yet the distance of each other from the opposite pole is so small that their powers counteract, and consequently destroy each other.
If, when a needle is attracted by the south pole of a magnet, a bar of iron be placed on the north pole, the needle is still more strongly attracted, because the iron acquires also a south pole, whose force is joined to that of the magnet.