429 



MAGNETO-ELECTRICITY. 



MAGNITUDE. 



430 



Many years ago the motions of a magnet under the influence of a 

 rotating copper-plate, known as " Arago's Rotations," excited consider- 

 able speculation and inquiry [MAGNETISM], until Faraday gave the true 

 explanation. The leading experiment may be shown by suspending a 

 bar magnet freely by its centre, so that it can move in a horizontal 

 plane, while beneath it a circular disc of copper being made to rotate, 

 the magnet will move round in the same direction as the disc, even 

 though a sheet of paper or of glass be interposed between them. 

 Discs of other metals on being rotated give motion to the magnet, but 

 none so readily as copper. A narrow strip cut out of the disc, from the 

 circumference to the centre, is sufficient to prevent the disc having 

 any action on the magnet when the former is made to rotate. If the 

 ctit edges of the disc be connected by a piece of wire, the power of 

 the disc over the magnet is completely restored. 



In investigating these phenomena, Faraday found that when a con- 

 ducting body is passed before the pole of a magnet, or between its 

 opposite poles, so as to cut the magnetic curves at right angles, electrical 

 currents are produced across the metal, transverse to the direction of 

 motion. If, for example, the copper disc c, be made to revolve in the 

 direction of the arrows at the edge, between the poles y s of a horse- 



shoe magnet, and a wire w, connected with one end of the galvano- 

 meter g, be pressed against the centre of the disc, while the other 

 wire v/ t proceeding from the galvanometer, rests against the edge of 

 the disc, between the magnetic poles, a current will flow from the 

 centre to the circumference of the disc, and then through the wires in 

 the direction of the arrows. If the disc revolve in the opposite direc- 

 tion the current will flow from the edge to the centre of the disc. 

 Now in Arago's experiment, when the copper disc revolves beneath the 

 magnet, it cute the magnetic curves at right angles (adopting Ampfcre's 

 view that a series of electric currents are perpetually circulating 

 around the component particles of a bar magnet, in planes at right 

 angles to the magnetic axis), in which case currents are produced 

 beneath the north pole from the centre of the plate towards the circum- 

 ference. These current* proceed from the circumference to the centre, 

 beneath the south pole, traversing the diameter of the plate, parallel 

 to the magnet, and return by the more distant parts of the plate. 

 Such currents exert a repulsive action on the magnet in a direction 

 coinciding with the motion, and there are no currents until the magnet 

 or the plate be in motion. 



More convenient arrangements than the above have been contrived 

 for the generation of magneto-electric currents, the best known of 

 which is Saxton's mof/aclo-electri'; machine. It consists of a powerful 

 horse-shoe magnet placed horizontally upon one of its sides, and oppo- 

 site and nearly in contact with its poles an armatu^ of soft iron u i is 

 made to revolve upon a horizontal axis A. In the figure the horse-shoe 

 magnet is not shown. The armature consists of two pieces of iron, 

 about 2 inches long, attached to a cross-piece of iron X, at such a dis- 

 tance as to be opposite the middle of each pole of the horse shoe 

 magnet. Each piece of iron, a 6, contains a coil, c d, of fine copper- 

 wire covered with silk to insulate the coils. The corresponding ends of 

 each of these coils are connected together ; one pair, e /, is attached to 

 the spindle , on which the armature revolves, and through it is con- 

 nected with a circular copper disc, i, the edge of which dips into a cup 



of mercury, ro, while the other pair of wires, g Ji, is connected with a 

 (tout piece of copper that passes through the axis of the spindle , 

 from which it is electrically insulated, and terminates in a slip of 

 copper, k, which u placed nearly at right angles to the cross-piece X. 

 Beneath k is a second cup of mercury, /, which can be made to com- 

 municate with the cup m by a wire or some other conductor. During 

 the revolution of the armature, the points of the slip alternately 

 dip into the mercury and rise above it. Now when I and m are con- 



nected, and the point k beneath the mercury, the circuit is complete. 

 The ends of the armature, a b, are exactly opposite the ends of the 

 horse-shoe magnet, but by giving motion to the axis they quit that 

 position, lose their magnetism, the current is broken, the slip k leaves 

 the mercury, and in doing so produces a bright spark. If the con- 

 nection between the mercury cups be made by means of wires, termi- 

 nating in copper cylinders, held one in each hand, a rapid succession of 

 powerful shocks will be felt on causing the armature to rotate. Water 

 and saline solutions may be decomposed by transmitting these currents 

 through them. For currents of high intensity, such as are required 

 for these decompositions, the armature should be furnished with a 

 great length of thin wire ; but for the display of large sparks, and the 

 ignition of platinum wire, &c., a shorter length of thicker wire may be 

 used. It should be observed that the currents produced during each 

 half revolution in the two limbs of the armature are in opposite direc- 

 tions, but. in some machines arrangements are made by means of com- 

 mutators [*TELEGRAPH, ELECTRIC] for keeping up a continuous current 

 in a uniform direction. This is of importance, since magneto-electric 

 machines now often take the place of the voltaic battery in the electric 

 telegraph, and in the process of electro-silvering, and electro-gilding 

 [ELECTRO-METALLURGY] ; and a powerful machine of this kind has 

 been employed by Mr. Holmes for producing a light of great steadiness 

 and intensity between two points of gas coke. This light has been 

 experimentally and successfully tried at the South Foreland Light- 

 house. 



MAGNETOMETER. A name applied to .those instruments, the 

 function of which is to measure any of the magnetic elements. These 

 instruments will be best described in connection with their subject, 

 namely, TERRESTRIAL MAGNETISM. 



MAGNIFYING POWER. [MICROSCOPE; TELESCOPE.] 



MAGNITUDE. This term is generally used synonymously with 

 quantity, and is sometimes even confounded with number. The dis- 

 tinction between the first two terms is not more marked than this : he 

 who answers the question " how much ? " describes the quantity, and 

 he who answers " how great ? " describes the magnitude. But since 

 magnitude is generally used in our language as applied to amount of 

 space, we may best describe our own idiom by laying down quantity 

 as the general term, and stating magnitude to mean usually the quan- 

 tity of space. The term however must be considered, in a mathe- 

 matical point of view, as originating with Euclid (whose word is 

 jie'70os), and it is used by him, not particularly as applied to space, 

 but also to everything which admits of .the introduction of the notion 

 of greater or less. In this sense, then, we have many magnitudes (all 

 moral qualities, for instance) which are not the object of mathematical 

 reasoning. So necessary is the notion of magnitude to our conception 

 even of things which we cannot measure, that we borrow idioms from 

 subjects within the province of mathematics. Thus we speak of force 

 of mind, and of it being greater in one individual than in another. 

 According to the definition of magnitude, namely, " that of which 

 greater or less can be predicated, when two of the same kind are com- 

 pared together," it follows that we include both mental as well as 

 material objects of conception. But the mathematics interpose the 

 postulate that no such object can be made matter of exact reasoning, 

 unless in cases which admit of the comparison being performed accord- 

 ing to some method the results of which shall be self-evident, and 

 inseparable from our notion of the thing measured. Let A and B be 

 two magnitudes of the same kind ; they are then, and then only, the 

 objects of mathematical comparison, when other magnitudes equal to 

 A and B can be found, and added together as often as may be desired ; 

 and when, moreover, any collection of A'S can be compared with a col- 

 lection of B'S, so as to ascertain which is greater or less than the other. 

 Angles furnish an instance of magnitude the conception of which is 

 exceedingly vague in the mind of most beginners, but which takes 

 precision and certainty in the course of mathematical study. Magni- 

 tudes, thus capable of comparison, are the objects of the doctrine of 

 PROPORTION. [See also NUMBER ; QUANTITY.] That part of geometry 

 which precedes proportion considers only the simple alternative of 

 equal or unequal, modes of inequality being necessarily deferred until 

 alter that consideration. 



By the magnitude of any bounded space the mathematician means 

 the results of measurement which will be -described in SOLID, &c., 

 DIMENSIONS ; but the common idiom refers to that which the mathe- 

 matician calls, for distinction, apparent magnitude. It is correct, in 

 the common meaning of the term, to say, that a man at a little distance 

 from the eye is larger than a remote mountain. In thus judging of 

 objects, the angles which they subtend at the eye furnish the means of 

 comparison. Experience, derived from the combination of sight and 

 touch, teaches us how to make those deductions which are necessary 

 before we can learn the absolute from the apparent magnitude. 



It is soon found that an object, as it recedes, grows smaller ; that is, 

 subtends a less angle. It is also seen that the recess is accompanied 

 by a loss of brightness and distinctness. The former is a consequence 

 of the loss of light wlu'ch takes place in its passage through the air ; 

 were it not for this, the same object would be equally bright at all 

 distances; for though the quantity of light which enters the eye is 

 diminished by increase of distance, yet the surface from which the 

 light appears to proceed is diminished in the same proportion. The 

 oss of distinctness is a consequence, first of the loss of light, next of 



