« AnteriorContinuar »
currents are in opposite directions. The attractions and tex: pulsions of these currents, unlike the mutual action of bodies electrized in the common way; take place equally in vacuo as in air.
The discovery of M. Oersted was limited to the action of the electric current on needles previously magnetized. But it was afterwards, and about the same time, ascertained both by Sir H. Davy, and M. Arago, that magnetism may be developed in steel not previously possessing it, by being placed in the electric current, and may even be excited in the uniting wire itself. Both philosophers ascertained; independently of each · other; that the uniting wire, becoming a magnet, attracts iron filings, and collects sufficient to acquire the diameter of a common quill. Thermoment the connexion is broken, all the fil. ings drop off; and the attraction diminishes also with the de.. caying energy of the pile. Filings of brass or copper, or wood sliavings, are not attracted at all.
The communication of magnetic properties to a steel needle, was effected, by Sir H. Davy and M. Arago, iu different ways. The former observed that steel needles, placed upon the coue necting wilt, bucanie:magnetic; those parallel to the wire act. ed like the wire itself; those placed across it each acquired two poles.. Such as were placed under the wire, the positive end of the battery being east, bad north poles on the south of the wire, and south poles to the north. The needles above were in the opposite directions, and this was constantly the case; whatever might be the.inclination of the needle to the wire. On breaking the connexion, the steel needles, placed across the uniting wire, retained their magnetism, while these placed parallel to it lost it at the moment. of disunion. Contact with the uniting wire was not found necessary, for the effect was produced though thick glass intervened..
Similar effects were produced in Sir H. Davy's experiments by the electricity excited by a common machine. A battery of 17 square feet, discharged through a silver wire 1- 20th of Electro-magnetism."
an inch diameter, rendered bars of steel, two inches long, and from 1-10th to ?-20th thick, so magnetic as to lift up pieces of steel wire and needles; and the effect was communicated to needles at the distance of five inches from the wire, even with the intervention of water or of thick plates of glass or metal.
On the suggestion of M. Ampere, M. Arago, in a different manner, also communicated magnetism to the needle, both by.. voltaic and common electricity:
Any wire, through which a current of electricity is passing, has a tendency to revolve round' a magnetic pole, in a plane perpendicular to the current; and that without any reference to the axis of the magnet, the pole of which is used. magnetic pole has a tendency to revolve round such a wire.
Suppose the wire perpendicular, its upper end positive, of attached to the positive pole of a voltaic battery, and its -lower end negative ; and let the centre of a watch dial repre- : sent the magnetic pole: if it be a north pole, the wire will rotate round it in the direction that the watch hands move;if it be: a. south pole, the motion will be in the opposite direction, From these two, the motions which would take place if the · wire were inverted, or the pole changed or made to move, may be readily ascertained; since the relation now pointed out remains constant. **
The theory of Oersted, which, though it appears to have led him to his principal discoveries, is not stated in a very intelligible manner, rests on the assumption of two different and opposite electricities, positive and negative, the former of which is developed by the more oxidizable, the latter by the less ox. idizable metal of galvanic arrangements. Each of these forces has a repulsive activity for itself, and an attractive activity for the opposite force. In the wire connecting the two opposite poles of a galvanic battery, and in the space around it, there
Several ingevious pieces of apparatus, have been invented to illustrate these Teets.
are, he supposes, two currents, the one of positive, the other of negative electricity, moving in spiral and opposite directions; and an effect is supposed to take place in the wire and around it, dependent on the union of these electricities, to which he gives the name of the electric conflict. By this conflict, all non-magnetic bodies appear to be penetrable, while, magnetic bodies, or rather their magnetic particles, resist its passage, and are therefore, moved by the impetus of the contending powers. All the effects on the north pole of the needle may be understood by supposing that negative electricity moves in a spiral line bent to the right, propelling the north pole, but not acting on the south pole. To positive electricity a contrary motion is ascribed, and a power of acting on the south pole, but not on the north. This theory requires, therefore, that there be two electric Auids : but in the opinion of Dr. Wollaston, which on every obscure topic of science is entiiled to the greatest deference, the phenomena may be equally well explained by a single electro-magnetic current, passing round the axis of the wire, in a direction determined by the position of the voltaic poles. The assumption of such a current is, it must be confessed, altogether gratuitous; but, without such a supposition, it is not easy to conceive any adequate cause for the motions that are observed in the magnetic needle, when, brought within the influence of the uniting wire.
ASTRONOMY. Remarks on Professor Struve's Observations to determine the Parallax of the fixed Stars. By J. Pond, Esq. Astr. Royal,
(From Brande's Journal.) Of the various attempts to discover the parallax of the fixed stars, the observations of Professor Struve must be regarded as among the best and most judicious. [Obs. vol. ii. iii]
His object is, by means of an excellent transit instrument furnished with seven wires, to determine the sum of the parale
Parallax of the Fixed Stars.
laxes of several fixed stars, differing nearly 12 hours in right ascension from each other.
The results which he obtains seem to verify a remark which I have often had occasion to make ; that in proportion as any improvement takes place either in our instruments or our processes the resulting parallax becomes proportionally less.
Of fourteeu sets of opposite stars thus compared, Mr. Struve finds seven, which give the parallax negative; this circumstance alone should suggest great caution in attributing to the effects of parallax the small positive quantities that are derived from the remaining seven.
Mr. Struve however is inclined to assign 0W. 16 of space as the parallax of ô Ursæ Minoris, and oʻ. 45 for the sum of the parallaxes of a Cygni, and · Ursæ Majoris. His learned coadjutor, M. Walbeck, who, it appears, has undertaken the calculations, is disposed to attribute the greatest portion of this parallax to the smaller star; a circumstance so improbable requires very strong evidence for its support.
But whatever reasonable doubt we may entertain as to* any one given result relating to such extremely minute quantities, yet the mean of the whole must be admitted to deserve very great confidence; and it is to this view of the subject (omitted by the learned author,) that I wish to direct the attention of Astronomers.
If we take the mean of the fourteen results as relating generally to stars from the 1st to the 4th magnitude, it will appear that the mean sum of the parallaxes of two opposite stars is equal to 0“. 036 of space, or the parallax of a single star equal to Oʻ.018.
If any reliance can be placed on these observations, every attempt to determine the parallax of these stars in declination must be entirely hopeless; since in this case we can only mea
* It should be remeinbered, that in a series of observations, it generally happens that some results will be erroneous by a greater quantity than the mean pro.. bable error.
Voyage of Discovery Irritability of the Tongue.
sure the shorter axis of the ellipse, and the uncertainty of refraction must amount, at least, to twenty times the quantity we are.in search.of.
Voyage of Discovery. Capt. Otto von Kotzebue is again about to circumnavigate "The world, having already been twice round it. expedition is appointed by the Russian government, and is well furnished with every thing that can promote its object. The object is rather to make accurate surveys than new discoveries, but an astronomer, mineralogist, and naturalist, from the University of Dorpat go with it, as well as other scieuti. fic men. The instruments are by Troughton and Jones, of Lone don.-Braride's Journal. Jan. 1824.
Blumenback on the Irritability of the Tongue. I had the tongue of a four year old ox which had been kill. ed in the common way, by opening 'the large vessels of tho neck, cut out in my presence while yet warm; and at the same time the heart, in order that I might compare the oscillatory motion of this organ, which is by far the most irritable that we are'acquainted with, with the motion of the tongue; and, when
I excited both viscera at the same time, by the same mecha**nical stimuli, namely, incisions with a knife and pricks of a
needle, the divided tongue appeared to all the by-standers to survive the heart more than seven minutes, and to retain the os· cillation of its fibres altogether for a quarter of an hour; and
80 vivid were the movements when I cut across the fore part of the tongue, that the butcher's wife compared them to those
of an eel in similar condition, quite in the way that Ovid kas "compared them to the mitions of the tail of a mutilated snake.
Edin. Phil. Jour. viii, 263.