Vickipedia

MAGNE’TO-ELECTRIC MACHINE (More recent forms of). Of late a new era has arrived in the construction of magneto-electric machines by the new forms of apparatus being marked by compactness, great simplicity in details, and marvelous power. The names chiefly associated with recent improvements are those of Wilde, Siemens, and Wheatstone, and Gramme of Paris. The machines described below, in which no permanent magnets are used, are now commonly called dynamo-electric machines, or, more shortly, dynamos.

Mr. H. Wilde, in 1866, patented a magneto-electric machine, founded on the principle that a current or a magnet indefinitely weak can be made to induce a current or a magnet of indefinite strength. Wilde’s original machine is shown in front elevation, fig. 1. It consists of two machines very similar to each other, the upper one MM’, and the lower EE’. The upper and smaller machine consists of sixteen permanent magnets, placed one behind the other. The front one only is seen. The poles of these are fixed at g, g (fig. 2), to what is termed the magnet cylinder. This consists of a hollow tube, made up of heavy masses of cast-iron, c, c, at each side, separated from each other by brass rods, &, 5, the whole being knit firmly together, above and below, by brass bolts at r, r’. The cast-iron side pieces thus form the poles

of the magnetic battery. The armature, which revolves within the tube of the magnet cylinder, is a long piece of soft iron, aa, and in section resembles an H. In the hollows of the H the wire is turned longitudinally. This armature is shown separately in fig. 3, part of the wooden tops which cover in the wire being removed to show how the wire is turned. This form of armature was first constructed by Siemens.

The ends of the armuture wire are soldered to two insulated iron rings, n, n’ (fig. 3), against which the springs, s, s (fig. 1), press, which convey the current from the revolving armature; m is the pulley of the driving-belt. If the cross-bar of the H stand upright (it lies horizontally in the figure), and the armature be turned round, while wires leading from the binding-screws, r, r’ (fig. 1), are connected with a galvanometer, it will be found that the current induced by the motion is in the same direction till the cross is again upright, but inverted. If the motion be continued beyond that point, a current in the opposite direction will

ensue, lasting till the cross-bar is in its first position. The right half of the armature gives off always one kind of electricity, and the left the other. The right and left springs, s, s, are thus always like poles, for they change from n to n’ (fig. 3), when the current in the armature changes. We come now to describe the singular peculiarity and merit of Wilde’s machine. The current got from the magneto-electric machine is not directly made use of, but is employed to generate an electromagnet some hundreds of times more powerful than the magnetic battery originally employed, by means of which a corresponding increase of electricity may be obtained. This electro-magnet J EE’ (fig. 1), forms the lower part of the figure, and by far the most bulky portion of the entire machine. It is of the horse-shoe form, E and E’ forming the two limbs of it. The core of each of these, shown by the dotted lines is formed by a plate of rolled iron, 36 inches in height, 26 inches in length, and 1 inch in thickness. Each is surrounded by a coil of insulated copper-wire (No. 10), 1650 feet long, wound round lengthwise in seven layers. The current has thus, in passing from the insulated binding-screw r, to the similar screw r’, to make a circuit of 3300 feet. Each limb of the electro-magnet is thus a flat reel of covered wire wrapped round a sheet of iron, the rounded ends alone of which are seen in the figure. The upright iron plates are joined above by a bridge, P, built up also of iron-plate, and are fixed below the whole way along with the iron bars «, «, to the sides of a magnet-cylinder of precisely the same construction as the one already described. The iron framework of the electro-magnet is shown by the dotted lines. The depth of the bridge is the same as the breadth of the bars »’, v’, which are of the same size as the bars v, v. The various surfaces of juncture in the framework are planed, so as to insure per-₁ feet metallic contact.

The upper and lower machine are in action precisely alike, only the upper magnet is a permanent magnet, and the lower one an electromagnet. We have the same magnet-cylinder, I, I, the same armature, A, and springs. S, S’, and the same poles, Z, Z’; the size is, however, different; the caliber of the magnet-cylinder is 7 inches. The diameter of the lower armature gives the name to the machine—viz., a 7-inch machine. Figs. 2 and 3 are on the scale of the lower machine (fig. 1). The length of wire on the lower armature is 350 feet. It is 35 inches in length, and is made to rotate 1800 times a minute. The cross framework attached at gg to the magnet-cylinder, in which the front journal, f, of the armature rotates (at Q), is shown in the lower machine (fig. 1). When the machine is in action, both armatures are driven simultaneously by belts from the same countershaft. For the electric light, the currents conveyed to the springs, S and S’, need not be sent in the same direction. In that case, the separation between n and n’ is vertical; and each spring presses against only one ring during the whole revolution, receiving and transmitting each revolution two opposite currents. Oil for the journal and commutator is supplied from the cup C.

A Wilde’s machine l 1/2 ton in weight, and 20 inches wide, driven by a steam-engine, produces a most brilliant electric light, and exhibits the most astonishing heating powers. Wheatstone and Siemens gave a new interpretation to Wilde’s principle. Their important discovery is of the following nature : Suppose the upper machine in fig. 1 removed, and that we have nothing but the electro-magnet and armature left. If the wires proceeding from the binding-screws of the armature be joined up with the electro-magnet, we might fancy that, there being no permanent magnetism, no result would follow on the armature being moved. Such, however, is not the case. If the armature be moved at any velocity, it will soon be brought to a halt by the mutual action ensuing. In the electro-magnet there is always some magnetism left. This induces a feeble current in the coil, but this is sufficient to make the magnet stronger and able to induce a stronger current, and this reciprocal action continues until it grows to an enormous intensity. So great indeed would it become, that if we had sufficient mechanical energy at our disposal to persist in the motion, the coils of armature and electro-magnet would be melted, and the machine destroyed. This startling discovery may, however, be thought of little value, as a machine that consumes its own electricity is of no external use. All machines now work on this reciprocal principle, and a description of them will best show how it is turned to account.

Ladd was the first to contract a machine on Wheatstone and Siemens’ principle. Fig. 4

(much smaller in scale than the preceding figures) gives a view of the armature. In it there are two coils, A and B; A the larger for furnishing the external current, B the smaller for exciting the electro-magnet. These two coils revolve together, the one at right angles to the other, in the same magnet-cylinder. In large machines he uses two magnet-cylinders, one at each end of the electro-magnet; or rather, he uses two electro-magnets and the two armatures complete the magnetic circuit. Ferguson of Edinburgh alters Ladd’s arrangement in using only one piece of iron for the armature of the machine with two grooves cut in it (fig. 5),

a larger one for the coil giving the external current, and a smaller one for the exciting current. This offers the advantage that the heating of the solid iron of the armature by repeated magnetism is lessened by being transformed into an electric current. The electro-magnet is thus fed by a current obtained not by an additional expenditure of energy, but by the utilization of force that would be otherwise converted into useless or even hurtful heat.

The great drawback of all the forms of the machine just described is the enormous velocity at which they rotate; some 2000 or more revolutions in the minute. At this speed a machine soon wears itself done. Another disadvantage is the heating of the armatures in Wilde and Ladd’s machine. Ferguson’s has never been tried on a large scale. It is found necessary to keep the armatures cool by a flow of cold water. This heat, however removed, is manifestly a mere squandering of the energy of motion, and a loss to the current given off. A third objection is the loss that always takes place when the side-springs change from the one ring to the other, sparks more or less bright accompanying the change. For the electric light, however, the alternate currents are used, and this source of loss is not experienced. These defects are removed in the latest form of the electro-magnetic machine by Gramme of Paris. In it, instead of a solid armature of iron, a ring is employed on which a great number of bobbins of wire are set. Fig. 6 is intended to explain the rudimentary principle of it. The ends of the wires of two contiguous bobbins are soldered to strips of metal called sectors. These are shown as radii in the figure.

In the machine itself they are brought down radially, then turned at right angles so as to be parallel to the axis of the machine. They are very numerous (though few in the figure), and being separated from each other by sheets of silk, form a compact whole. Metallic brushes, B, B, rub on the end face of the sectors, and form the poles of the revolving armature. The principal of action may be thus understood. Suppose we first ascertain what takes place in the coil of one bobbin as it revolves in the presence of the magnetic poles, P, N- If we start from the equatorial line, EE’,

and go by successive impulses, we find that, when the bobbin is joined with a galvanometer, the current induced is always in one direction until we come again to the equatorial line; but when we pass this, the current is reversed on the other side- This is much the same as what is found in the Siemens armature. But there is this difference here. The armature wire with the sectors is continuous from end to end. On each side of the equatorial line, we have two equal and opposite electric forces or batteries, and these, if left alone, would neutralize each other. But if, in the equatorial line, we introduce brushes to act as poles, we have, as it were, two galvanic batteries joined up, as it is called, in quantity, with both positive poles together and also both negative. The brushes embrace several sectors at once, so there is no spark when they leave any particular sector, contact being established with the others. The conditions of the machine never alter, and hence the current is perfectly steady, and the sectors being always of the same sign at the points where the brushes rub, the current is always in the same direction. Siemens and Wheatstone’s principle is employed in Gramme machines. There are two fixed electro-magnets, and two armatures on the same spindle; one electro-magnet and one armature being set apart for exciting both electro-magnets, and the other armature and electro-magnet for sending out the external current. Astonishing as were the effects produced by Wilde’s machine, those obtained from Gramme’s seem quite to eclipse them. In comparing two magneto-electric machines, we must take into account the kind of wire used for the revolving armature. For tension purposes, a thin and long wire gives the best results; for quantity or heating purposes, a short and thick wire does best.

To compare a tension with a quanity armature, the same test even in the same machine would give most contradictory results. But comparing, so far as possible, machines intended for the same purpose, Gramme seems to have the advantage of all others. In the first place, the speed of revolution seldom exceeds 800 revolutions per minute; 300 is sufficient for most purposes. A Gramme machine driven by the hand will melt 10 inches of an iron wire 1/28 of an inch in diameter, a feat not accomplished by any other arrangement. A Gramme machine adapted for electro-plating, and worked by a 1 horse-power engine, deposits nearly 27 oz. of silver per hour, an achievement for transcending the similar performance of other machines. Among the heating wonders of the Gramme machine we are told of a file half an inch in diameter being burnt up in 5 minutes, of 15 feet of No. 18 platinum wire being brought to a glowing heat, and of 8 feet of iron wire .051 inch in diameter being fused. Many other forms of dynamo-machines are now in use, depending on the principles already explained; the chief being the Siemens machine, the Brush, the Bur-gin, and Edison’s. Dynamo-machines for producing current electricity have been largely applied to produce the electric light (see ELECTRIC LIGHT).Machines of this kind are most sucessfully employed in the most improved method of electro-metallurgy.

ELECTRICITY,THE THEORY OF.What is electricity? is a problem still unsolved. Modern research has, however, indicated the lines along which increasing knowledge will probably lead to the true solution. The two aspects of this development, the mathematical and the experimental, call for a brief consideration.

The great advance in the mathematical treatment is clue to the explicit introduction of the idea of the potential. This, as far as electrical science is concerned, we owe to Green, in whose memoirs (published 1828) we have the potential first so named; for though Laplace, in his Mécanique Celeste, had already pointed out its properties, yet to him and his immediate successors, it was merely a mathematical function, from which the forces of a system could be easily derived. Green, however, lived before his time. His memoir was not appreciated; and not until his theorems had been re-discovered independently by Gauss (1839), Charles, Sturm, and (more especially) Thomson, was his Essay on the Application of Mathematical Analysis to the Theories of Electricity and Magnetism brought before the scientific world by the last-named physicist. This was in 1845; and since that date the mathematical development of electricity has gone on rapidly along the lines so clearly laid down by Green. To Thomson, more particularly, is the present advanced state of the subject clue; his method of electric images being especially worthy of mention. See his Reprint of Papers on Electrostatics and Magnetism (1872).

Meanwhile, in the hands of Faraday, the whole method of regarding electrical phenomena was being completely revolutionized. The older electricians had looked upon the charged conductor as the real seat of electrical action; Faraday was led by his classical researches to the conclusion, that this was to be sought for in the dielectric or medium separating the conductors. Through this medium, surrounding a given charged body, Faraday imagined lines of electric force drawn. These lines of force he conceived to start perpendicularly from the surface of the body, and to determine the direction, and, by their closeness, the magnitude of the force which would act at any given point upon a small quantity of electricity placed there. Such a region, with its lines of force, Faraday termed an electric field; and, in a precisely similar way, he treated the lines of magnetic force which he imagined to emanate from a magnet. The mathematicians could not at first accept Faraday’s view, as they imagined their analysis could be based solely upon the conception of action at a distance, to which Faraday’s views are opposed. Doubtless, in the first instance, the equations were established directly upon this hypothesis; but as pointed out in an early paper of Thomson’s (see Reprint, page 29), Faraday’s conceptions, expressed in appropriate mathematical language, will lead to the same results as far as analysis is concerned.

Faraday, indeed, was a mathematician in the truest sense of the word, and his method of representing electrical phenomena is in-intrinsically mathematical. But to give appropriate mathematical expression to these conceptions of action through a medium, required the genius of Clerk Maxwell, who, in his classical treatise on electricity and magnetism, has done more perhaps than any other man to clear the way of false notions. His treatment of electro-static induction, as developed from Faraday’s point of view, is very lucidly given in the opening chapters of the Elementary Treatise on Electricity (1881), which was left by its author in an unfinished state. In it the idea of the potential is ever present The notion of equipotential surfaces, with the tubes or lines of force cutting them at right angles, is made to yield, by a simple synthesis, theorems that were long supposed to be demonstrable only by means of abstruse analysis. Still following the lead of Faraday, Maxwell has, in his larger treatise, elaborated a theory of the mechanical action through the dielectric. He has shown that the hypothesis of a tension along the line of force at any point proportional to the square of the resultant electromotive force at the point, together with an equal pressure in directions at right angles, gives dynamical effects identical with those given by the ordinary theory of action at a distance.

Previous to the discoveries of Faraday relating to the induction of currents, the laws of electro-kinetics had been fully established, and the mathematical treatment given by Ampere. When Faraday’s important researches were published, it became expedient to give a satisfactory theory from which these phenomena might be deduced. Weber’s theory of the particles of electricity exerting a mutual force depending upon their relative motion, was such an attempt. The mere fact that the laws of induction could be deduced from it, gives the theory no claim to be a physical truth. For Helmholtz and Thomson had demonstrated that Faraday’s laws of induction were a necessary consequence of the truth of Ampere’s phenomena, taken in conjunction with the principle of the conservation of energy. Weber’s hypothesis includes Ampere’s, and is at the same time consistent with the conservation principle; and any other hypothesis satisfying these conditions would necessarily lead to the same results. Now, Weber’s hypothesis is essentially one of action at a distance, the conductor carrying the current being the seat of the action; whereas, Faraday always looked beyond the conductor to the surrounding region, and asked himself what was going on there. Thomson early suggested that the kinetic energy of a current was the energy of vortical motions in the space surrounding the conductor; and this idea has been elaborated by Maxwell into his ingenious electro-magnet theory of light. The medium that transmits light is supposed to be the medium through which magnetic and electric action is propagated. One definite result of the theory as developed by Maxwell is, that the square root of the specific inductive capacity of a dielectric is equal to its refractive index for light of infinite wave length—a remarkable result, which is wonderfully borne out by Silow’s and Boltzmann’s experiments.

Also, the curious equality between the velocity of light, and the velocity which expresses the ratio of the electro-static to the electro-magnetic unit of electricity is, to say the least, not unfavorable to the theory. The special hypothesis on which Maxwell bases his electro-magnetic theory of light does not, of course, affect the beautiful manner in which he derives, from the conception of a medium, the dynamical equations of magneto-electric induction. By the application of Lagrange’s general equations to the moving system, he develops the known laws of induction and the mechanical action of currents.

The close connection between electricity and magnetism requires us to mention, in conclusion, the splendid mathematical investigations of Sir William Thomson in the latter subject. Nearly one-half of the Reprint is taken up with his papers on magnetism, which, in power and logical sequence, have rarely been equalled. The theory of induction in crystalline and non-crystalline substances, the whole question of magnetic permeability, the distinction between paramagnetic and diamagnetic bodies, the various theoretical distributions of magnetism, the mutual action of magnets, the theory of electro-magnets, may be noted specially, inasmuch as they involve some of the deepest problems yet imagined in physical science.