mean time come into view; the effect of the iron in the ship upon the Compass. And this has gone on increasing as guns, cables, stays, knees, have been made of iron: then steam-engines with funnels, wheels, and screws, have been added; and finally the whole ship has been made of iron. How can the compass be trusted in such cases? I have already said in the history that Mr. Barlow proposed to correct the error of the compass by placing near to the compass an iron plate, which from its proximity to the compass might counterbalance magnetically the whole effect of the ship's iron upon the compass. This correction was not effectual, because the magnetic forces of the plate and of the ship do not change their direction and value according to the same law, with the change of position. I have further stated that Mr. Airy devised other means of correcting the error. I may add a few words on the subject; for the subject has been further examined by Mr. Airy and by others. It appears, by mathematical reasoning, that the magnetic effect of the iron in a ship may be regarded as producing two kinds of deviation which are added together;-a 'polar-magnet deviation,' which changes from positive to negative as the direction of the ship's keel, in a horizontal revolution, passes from semicircle to semicircle; and a 'quadrantal deviation,' which changes from positive to negative as the keel turns from quadrant to quadrant. The latter deviation may be remedied completely by a mass of unmagnetized iron placed on a level with the compass, either in the athwartship line or in the fore-and-aft line, according to circumstances. The polar-magnet deviation' may be corrected at any given place by a magnet or magnets, but the magnets thus applied at one place will not always correct the deviation in another magnetic latitude. For it appears that this deviation arises partly from a magnetism inherent in the materials of the ship, not changing with the change of magnetic position, and partly from the effect of terrestrial magne 8 Phil. Truns. 1856. tism upon the ship's iron. But the errors arising from both sources may be remedied by adjusting, at a new locality, the positions of the corrective magnets. The inherent magnetism of the ship, of which I have spoken, may be much affected by the position in which the ship was built; and may change from time to time; for instance, by the effect of the battering of the waves, and other causes. Hence it is called by Mr. Airy sub-permanent magnetism.' Another method of correcting the errors of a ship's compass has been proposed, and is used to some extent; namely, by swinging the ship round (in harbour) to all points of azimuth, and thus constructing a Table of Compass Errors for that particular ship. But to this method it is objected that the Table loses its value in a new magnetic latitude much more than the correction by magnets does; besides the inconveniences of steering a ship by a Table. BOOK XIII. VOLTAIC ELECTRICITY. FARADA CHAPTER VIII. MAGNETO-ELECTRIC INDUCTION. ARADAY'S discovery that, in combinations like those in which a voltaic current was known to produce motion, motion would produce a voltaic current, naturally excited great attention among the scientific men of Europe. The general nature of his discovery was communicated by letter to M. Hachette at Paris, in December, 1831; and experiments having the like results were forthwith made by MM. Becquerel and Ampère at Paris, and MM. Nobili and Antinori at Florence. It was natural also that in a case in which the relations of space which determine the results are so complicated, different philosophers should look at them in different ways. There had been, from the first discovery by Oersted of the effect of a voltaic current upon a magnet, two rival methods of regarding the facts. Electric and magnetic lines exert an effort to place themselves transverse to each other (see chapter iv. of this Book), and (as I have already said) two ways offered themselves of simplifying this general truth:to suppose an electric current made up of transverse magnetic lines; or to suppose magnetic lines made up of transverse electric currents. On either of these assumptions, the result was expressed by saying that like currents or lines (electric or magnetic) tend to place themselves parallel; which is a law more generally intelligible than the law of transverse position. Faraday had adopted the former view; had taken the lines of magnetic force for the fundamental lines of his system, and defined the direction of the magneto-electric 1 Ann. de Chimie, vol. xlviii. (1831), p. 402. current of induction by the relation of the motion to these lines. Ampère, on the other hand, supposed the magnet to be made up of transverse electric currents (chap. vi.); and had deduced all the facts of electrodynamical action, with great felicity, from this conception. The question naturally arose, in what manner, on this view, were the new facts of magneto-electric induction by motion to be explained, or even expressed? Various philosophers attempted to answer this question. Perhaps the form in which the answer has obtained most general acceptance is that in which it was put by Lenz, who discoursed on the subject to the Academy of St. Petersburg in 1833.2 His general rule is to this effect: when a wire moves in the neighbourhood of an electric current or a magnet, a current takes place in it, such as, existing independently, would have produced a motion opposite to the actual motion. Thus two parallel forward currents move towards each other: hence if a current move towards a parallel wire, it produces in it a backward current. A moveable wire conducting a current downwards will move round the north pole of a magnet in the direction N., W., S., E.:-hence if, when the wire have in it no current, we move it in the direction N., W., S., E., we produce in the wire an upward current. And thus, as M. de la Rive remarks,3 in cases in which the mutual action of two currents produces a limited motion, as attraction or repulsion, or a deviation right or left, the corresponding magneto-electric induction produces an instantaneous current only; but when the electrodynamic action produces a continued motion, the corresponding motion produces, by induction, a continued current. Looking at this mode of stating the law, it is impossible not to regard this effect as a sort of reaction; and accordingly, this view was at once taken of it. Professor Ritchie said, in 1833, 'The law is founded on the universal principle that action and reaction are equal.' Thus, if voltaic electricity induce magnetism Acad. Petrop. Nov. 29, 1833. Pogg. Ann. vol. xxxi. p. 483. under certain arrangements, magnetism will, by similar arrangements, react on a conductor and induce voltale electricity. There are still other ways of looking at this matter. I have elsewhere pointed out that where polar properties co-exist, they are generally found to be connected, and have illustrated this law in the case of electrical, magnetical, and chemical polarities. If we regard motion backwards and forwards, to the right and the left, and the like, as polar relations, we see that magneto-electric induction gives us a new manifestation of connected polarities. Diamagnetic Polarity. But the manifestation of co-existent polarities which are brought into view in this most curious department of nature is not yet exhausted by those which we have described. I have already spoken (chap. iv.) of Dr. Faraday's discovery that there are diamagnetic as well as magnetic bodies: bodies which are repelled by the pole of a magnet, as well as bodies which are attracted. Here is a new opposition of properties. What is the exact definition of this opposition in connexion with other polarities? To this, at present, different philosophers give different answers. Some say that diamagnetism is completely the opposite of ordinary magnetism, or, as Dr. Faraday has termed it for the sake of distinction, of paramagnetism. They say that as the north pole of a magnet gives to the neighbouring extremity of a piece of soft iron a south pole, so it gives to the neighbouring extremity of a piece of bismuth a north pole, and that the bismuth becomes for the time an inverted magnet; and hence, arranges itself across the line of magnetised force, instead of along it. Dr. Faraday 4 On the Reduction of Mr. In the second edition of this Faraday's discoveries in Magneto- history I used the like expres electric Induction to a General sions. Law. Trans. of R. S. in Phil. Mag. N.S. vol. iii. 37, and vol. iv. p. 11. 5 Phil. Ind. Sc. B. v. c. ii. |