ON PHYSICS, OR NATURAL PHILOSOPHY. (Continued from page 444.) TERRESTRIAL MAGNETISM-THE COMPASS. Inclination or Dip; Magnetic Equator. From the fact that the compass affects the direction towards the north, it might be supposed that the force which acts upon it comes from a point in the horizon; but this is not the case, for if we arrange the needle in such a manner that it can move freely in a vertical plane about a horizontal axis, we shall find that though the centre of gravity of the needle exactly coincides with the axis of suspension, its south pole inclines constantly to the north pole of the earth in this quarter of the globe. In the southern hemisphere, on the contrary, it is the north pole of the needle which inclines towards the south pole of the earth. When the vertical plane in which the needle moves coincides with the magnetic meridian, the angle it makes with the horizon is called the inclination or dip. In any other plane than the magnetic meridian, the dip increases, and is ninety degrees in a plane perpendicular to the magnetic meridian. In fact, the magnetic force of the earth being then resolved into two forces, the one vertical and the other horizontal, the former gives the needle its vertical position, while the latter, acting in the direction of the axis of suspension, cannot turn the needle. The inclination, like the declination, varies with the latitude, but according to a more fixed law. Near the north pole it is nearly ninety degrees, and proceeding thence towards the equator, it diminishes till it becomes nothing at the equator. In the southern hemisphere the inclination reappears, but in a contrary direction, that is to say, it is the north pole of the needle which sinks below the horizon. The curve which passes through all the points at which the inclination is nothing is called the magnetic equator, and the points at which it is 90° are called magnetic poles. According to the observations of M. Duperrey, the magnetic equator appears to intersect the terrestrial equator in two diametrically opposite points, one in the Pacific and the other in the Atlantic Ocean. The inclination varies at the same place from one period to another. The Inclination Compass is the name given to an instrument used for measuring the magnetic inclination. This compass, which is made entirely of copper, is composed first of a horizontal circle m, graduated, and supported by three screws, fig. 370. Above this circle is a plate A, moveable about a vertical Fig. 370. axis, and supporting by means of two columns a second graduated circle M, which measures the inclination. A frame r supports the needle a b, and a level n serves by means of the three screws to give a horizontal position to the diameter passing through the two zeros of the circle M. To observe the inclination, we begin by determining the magnetic meridian, which is done by turning the plate a upon the circle m till the needle becomes vertical, which takes place meridian. Then turning the plate a through 90 degrees on when the needle is in a plane perpendicular to the magnetic the circle m, we bring the vertical circle M to the magnetic meridian. The angle dea which the magnetised needle then makes with the horizontal diameter, is the angle of inclination. But there are two causes of error which must be taken into account. First, the magnetic axis of the needle may perhaps not coincide with the axis of the needle itself, whence arises an error which is corrected by the method of turning already described. Secondly, the centre of gravity of the needle may possibly not coincide with the axis of suspension, and then the angle de a is too small or too large, according as the centre first case the action of gravitation is contrary to that of terresof gravity is above or below the centre of suspension, for in the trial magnetism in turning the needle, while in the second it is in the same direction. This error is corrected by reversing the poles of the needle, which is accomplished by rubbing it with the contrary poles of two bars in such a manner, that each pole of the magnet is rubbed by a pole of the same name as its own. The direction of the needle being then changed, if its centre of gravity was before above the point of suspension, it is now below, and the angle of inclination which was before too small is now too large. The true value is found by taking the mean between the two. Astatic Needle and System.--A needle withdrawn from the magnetic action of the earth is said to be astatic. This is the case with a needle moveable about an axis in the plane of the magnetic meridian in a direction parallel to the inclination. An astatic system is the combination of two needles of the same power placed parallel, with their contrary poles opposite each other, as seen in fig. 371. If the two needles are of exactly VOT. V. the same power, the contrary action of the earth upon the poles a' and b, as well as on a and b, is self-destructive, and the system is completely astatic. MAGNETISATION AND LAWS OF MAGNETIC ACTION. Sources of Magnetisation.-The various sources of magnetisation are the influence of powerful magnets, terrestrial magnetism, and electricity. There are three methods of magnetising by magnets, simple touch, separate touch, and double touch. Whichever of these methods be employed, there is a limit to the degree of acquired magnetic power, depending upon the temper and intensity of the magnets employed. When this limit is attained, the bar is said to be magnetised to saturation. When it has been passed, the bar soon returns to it, and even tends to sink below it, if the magnetic power is not retained by means of a process to be described presently. Method of Simple Touch.-This consists in sliding the pole of a powerful magnet from one end of the bar we wish to magnetise to the other, and repeating the operation several times, but always in the same direction. This process is incapable of producing any great effect, and consequently is only applicable to small bars. It has the further inconvenience of frequently developing consequent points. 134 458 Method of Separate Touch.-This was adopted by Knight in this country in 1715, and consis's in placing the two contrary poles of two bars of equal intensity at the middle of the bot s wish to magnetise, and making them both slide simultaneously towards one of the ends of the bar, holding them vertically. Eich magnet is then brought back to the middle of the bar, and the operation is repeated several times on both sides till the bar is magnetised. Duhamel improved this method by placing the two ends of the bar to be magnetised at the contrary poles of two fix d magnets, whose action combines with that of the moveable magnets, the relative position of the poles being the same as in fig. 372. This process gives the most steady magnetic power. Method of Don'e Tch-In this meth, introduced by Mitchell, the two magnets en ployed in friction are still placed at the middle of the bar we wish to mag tise, with their contrary poles towards each other; but instead of sliding towards its extremities in contrary tons, they are kep' at by a small piece of wood a certain distance from e? bher from the middle to the tar extremity, and so on be bar undergoes the same observed that in the the magnets lose none of their power, which proves si de magnetic fluids do not pass -The action of the earth up. magress being similar to that of placed between them, and ode the one extremity, therm in such a manner that ca. 5. number of these operations various processes. straight line (fig. 371). The bundle represented in fig. 373 is formed of five plates of steel placed side by side, that in fig. 374 consists of twelve plates arranged in three layers with four plates in each. The form of the horse-shoe is preferabl Fig. 374. for supporting a weight by means of the magnet, because use is made of both poles at the same time. The magnetic power of a bundle is not equal to the sum of the separate forces of the bars, owing to the repulsive action of neighbouring poles upon one another. Armatures or Keepers of magnets, are the pieces of soft iron which are placed in contact with the poles, to preserve or increase the magnetic power. Fig. 375 represents a natural magnet with its armatures. On the surfaces corresponding to the poles are two plates of soft iron, each terminated by a massive block. Under the influence of the natural magnet these plates become magnetised, and the letters A and B representing the position of the poles of the natural magnet, it is easy to see that those of the armatures are respectively represented by ab. Now these armatures, when once they are Fig. 372. asm constantly tends to separate the a neutral state in soft iron and steel. tarce, the coercive force being very the earth is insufficient to produce magmot so with a bar of soft iron, especially if it Actie meridian parallel to the inclination. an unsteady sort of magnetisation. Armatures of Magnets,-A magnetic bude is a ciecon of magnetised bars united parallel to Sometimes it is each other at their poles of the same name. made in the shape of a horse-shoe (fig. 373), sometimes of a go on increasing till they have reached a certain limit that is never passed. The substance supported a'b', which is of soft iron, acts as a second armature, for being magnetised by induction, its poles a' and ' react upon the poles a and b of the former. To furnish artificial magnets with armatures, place them in pairs, as represented in fig. 376, with the contrary poles opposite each other; then put two small bars of soft iron A B in connexion with the poles, these becoming magnetised by induction, their poles react upon the magnetised bars and preserve their intensity. As for moveable needles (figs. 362 and 365) as they turn towards the magnetic poles of the globe, the influence of this latter acts as an armature. Law of Magnetic Attraction and Repulsion.-Coulomb was the first who established the law, that magnetic attraction and repulsion is inversely as the square of the distance, a principle which he established by two methods, that of the torsion balance and that of oscillations. Fig. 376. Torsion Balance.-This apparatus consists of a glass box (fig. 377), with a glass cover capable of being removed at pleasure, and having an opening near the edge to admit a magnet a b. In the centre of this cover is a second opening, into which a glass tube is fitted so as to turn with slight friction against the edges of the orifice. This tube has in the upper part of it a micrometer, or combination of two pieces, one of which D is fixed and divided round its edge into 360 degrees, and the other E which is moveable, has graduations marked on it, to show the number of degrees through which it is turned on the dial-plate D. On the left of the figure at e and d the two pieces which constitute the micrometer are represented on a large scale. To the disc e are affixed two uprights, through which passes a horizontal axis. Upon this axis a very fine silver thread is wound, supporting a magnetised Fig. 377. needle AR. Lastly, at the bottom of the box is a dial-plate with divisions to measure the displacement of the needle A B and consequently the torsion of the silver thread, or the force with which it returns to a state of rest. The index a of the disc e being at zero on the dial d, the box is placed in such a manner that the centre and the zero of the lower dial-plate may be in the magnetic meridian. Then, taking the needle A B away, replace it by a similar needle made of copper or any other non-magnetic metal. Turn the glass tube, and with it the pieces E and D in such a way that this needle may stop at the zero point of the lower dialplate. The magnetised needle not being yet inserted, remove the non-magnetic needle, and put the magnetised needle A B in its place, which will then be exactly in the magnetic meridian, and the torsion of the silver thread is null. The apparatus being thus arranged, it is necessary before introducing the magnet ab to know the action of the earth upon the moveable needle A B when it is a certain number of degrees out of the magnetic meridian. For this purpose, turn the piece в till the needle A B is moved one degree in the same direction. The number of degrees minus one, through which the micrometer has been turned, represents the total torsion of the thread. In the experiments made by Coulomb, this number was 35, but it varies with the length of the thread, its diameter, and the intensity of the bar A B. Now the needle remaining at present in equilibrium, it is evident that the force of the torsion of the thread is precisely equal and opposite to the directing influence of the earth. This action, then, in the experiments of Coulomb was represented by 35 for a deviation of one degree; but the force of torsion being proportional to the angle of torsion (vol. iv. p. 101), and the directing action of the earth, when there has once been equilibrium, being equal to it; it follows that this latter force, for the deviation of 2°, 3°, etc., is represented by twice, three times, etc. 35 degrees. The action of the earth being determined, put the magnet a b into the box, taking care to put poles of the same name The pole of A of the needle is then opposite each other. repelled, and if N represent the number of degrees which measure the angle of deviation when the needle A B is in equilibrium, this needle tends to return to the magnetic meridian with a force represented by N+ 35 N, the part N being due to the torsion of the thread, and the other part 35 N to the action of the earth. But since it does not return to the magnetic meridian, the repulsive force exercised between the poles a an A must be itself equal to N + 35 N. Now turn the disc E in such a way that the angle of deviation N may become half what it was. According to the position of the needle A B in the accompanying figure, it would be necessary to turn it from right to left. Representing the displacement of the disc E by n, we see that the suspension thread is twisted n degrees to the left at its upper end, and degrees to the right at its N magnetic meridian is ( n + 1 ) + 2 "+representing the action of the earth. Now, N quadruple of the quantity N + 35 N obtained by the first experiment. Therefore the law of Coulomb is demonstrated, for experiments are made with arcs, N and, so small as to be nearly equal to their chords, that is to say, that when the arc is bisected, a A, the distance between the two poles is also apparently bisected. Method of Oscillations.-This consists in making a magnetised needle oscillate in equal times, first under the sole influence of the earth, and then under the combined influence of the earth and the attracting pole of a magnet placed at two unequal distances one after the other. From the three numbers of oscillations observed, the law of Coulomb may be deduced by calculation. Measure of Terrestrial Magnetism.-A great number of philosophers and navigators have employed themselves in measuring the magnetic intensity of the earth in different places and at different periods. Several methods have been adopted, which consist in making a needle of inclination or declination oscillate for a given time, and then deducing the relative intensities from the number of oscillations. Their observations have led to the establishment of the following laws. 1. The intensity of the earth's magnetism increases as we recede from the equator, and it appears to be half as great Method of Separate Touch.-This was adopted by Knight in this country in 1745, and consists in placing the two contrary poles of two bars of equal intensity at the middle of the bar we wish to magnetise, and making them both slide simultaneously towards one of the ends of the bar, holding them vertically. Each magnet is then brought back to the middle of the bar, and the operation is repeated several times on both sides till the bar is magnetised. Duhamel improved this method by placing the two ends of the bar to be magnetised at the contrary poles of two fixed magnets, whose action combines with that of the moveable magnets, the relative position of the poles being the same as in fig. 372. This process gives the most steady magnetic power. Method of Double Touch.-In this method, introduced by Mitchell, the two magnets employed in friction are still placed at the middle of the bar we wish to magnetise, with their contrary poles towards each other; but instead of sliding towards its extremities in contrary directions, they are kept at a certain distance from each other by a small piece of wood placed between them, and slide together from the middle to one extremity, then from this to the other extremity, and so on in such a manner that each half of the bar undergoes the same number of these operations. It is to be observed that in the various processes of magnetisation the magnets lose none of their power, which proves that the magnetic fluids do not pass from one bar to another, straight line (fig. 371). The bundle represented in fiz. 37 formed of five plates of steel placed side by side, that i 374 consists of twelve plates arranged in three layers four plates in each. The form of the horse-shoe is pret Fig. 374. for supporting a weight by means of the magnet. Armatures or Keepers of magnets, are the pic Magnetisation by the Action of the Earth.-The action of the earth upon magnetic substances being similar to that of magnets, terrestrial magnetism constantly tends to separate the two fluids which are in a neutral state in soft iron and steel. But in the latter substance, the coercive force being very great, the action of the earth is insufficient to produce magnetisation. It is not so with a bar of soft iron, especially if it is placed in the magnetic meridian parallel to the inclination. Yet even this is only an unsteady sort of magnetisation. Magnetic Bundles. Armatures of Magnets,-A magnetic bundle is a collection of magnetised bars united parallel to each other at their poles of the same name. Sometimes it is made in the shape of a horse-shoe (fig. 373), sometimes of a Fig. 373. magnetised, react in their but with them they b ve lf in y can funda ed. When we think of that liberty ee agency, and to the performance of kind of liberty which we have in our the moral quality of an act, we never k than the spontaneous inclination of nk it necessary to know in what way quired. If the action proceed from is concerned it is a moral act. We greater or more desirable liberty than atures, indeed, cannot possess that inden is the prerogative of the Deity. The ding his liberty, is still under the govern.dence. it that we entertain distinct and accurate the common use of terms, and according to the the prejudice immediately arises that it cannot it to satisfy us, adduced; yet it d, there are any ,ney of man. It is Pposition to any other reason and the evident erience seem to stand in arise from some misappreir understanding is given us 1, no proposition clearly pertively or by ratiocination, can er truth. efore, in the first place, to have neant by liberty, and what by nce must be, not to metaphysical mon judgment and clear conviction It has already been stated that that sary to moral agency, can be nothing of doing what we will, to the extent of reedom of action in conformity with our When a man is compelled by force to strike n not by the force of strong motives, but by force), we say he is not accountable, because Let the distinction between what is certain and what is necessary be fully comprehended and attended to, and a great part of the darkness which, in the view of many, has obscured this subject will be dissipated. Although, then, it should be demonstrated that the will is as certainly governed by motives as the scale of the balance is by weights, yet there can be no legitimate inference from the one to the other, as if that would prove that the will is not free but under a necessity. The difference lies, not in the difference of certainty in the two cases, but in the difference in the nature of the causes of that certainty. |