Online Encyclopedia

ELECTROMAGNETISM

Online Encyclopedia
Originally appearing in Volume V09, Page 229 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
ELECTROMAGNETISM, that branch of physical science which is concerned with the interconnexion of electricity and magnetism, and with the production of magnetism by means of electric currents by devices called electromagnets. History.—The foundation was laid by the observation first made by Hans Christian Oersted (1777-1851), professor of natural philosophy in Copenhagen, who discovered in 1820 that a wire uniting the poles or terminal plates of a voltaic pile has the property of affecting a magnetic needle 1 (see ELECTRICITY). 1 In the Annals of Philosophy for November 1821 is a long article entitled " Electromagnetism " by Oersted, in which he gives a detailed account of his discovery. He had his thoughts turned to it as far back as 1813, but not until the loth of July 182o had he actually made his discovery. He seems to have been arranging a compass needle to observe any deflections during a storm, and placed near it a platinum wire through which a galvanic current was passed. Oersted carefully ascertained that the nature of the wire itself did not influence the result but saw that it was due to the electric conflict, as he called it, round the wire; or in modern language, to the magnetic force or magnetic flux round the conductor. If a straight wire through which an electric current is flowing is placed above and parallel to a magnetic compass needle, it is found that if the current is flowing in the conductor in a direction from south to north, the north pole of the needle under the conductor deviates to the left hand, whereas if the conductor is placed under the needle, the north pole deviates to the right hand; if the conductor is doubled back over the needle, the effects of the two sides of the loop are added together and the deflection is increased. These results are summed up in the mnemonic rule: Imagine yourself swimming in the conductor with the current, that is, moving in the direction of the positive electricity, with your face towards the magnetic needle; the north pole will then deviate to your left hand. The deflection of the magnetic needle can there-fore reveal the existence of an electric current in a neighbouring circuit, and this fact was soon utilized in the construction of instruments called galvanometers (q.v.). Immediately after Oersted's discovery was announced, D. F. J. Arago and A. M. Ampere began investigations on the subject of electromagnetism. On the 18th of September 1820, Ampere read a paper before the Academy of Sciences in Paris, in which he announced that the voltaic pile itself affected a magnetic needle as did the uniting wire, and he showed that the effects in both cases were consistent with the theory that electric current was a circulation round a circuit, and equivalent in magnetic effect to a very short magnet with axis placed at right angles to the plane of the circuit. He then propounded his brilliant hypothesis that the magnetization of iron was due to molecular electric currents. This suggested to Arago that wire wound into a helix carrying electric current should magnetize a steel needle placed in the interior. In the Ann. Chime (1820,. 15,p.94), Arago published a paper entitled "Experiences relatives a 1'aimantation du fer et de 1'acier par faction du courant voltaique," announcing that the wire conveying the current, even though of copper, could magnetize steel needles placed across it, and if plunged into iron filings it attracted them. About the same time Sir Humphry Davy sent a communication to Dr W. H. Wollaston, read at the Royal Society on the 16th of November 182o (reproduced in the Annals of Philosophy for August 1821, p.81), On the Magnetic Phenomena produced by Electricity," in which he announced his independent discovery of the same fact. With a large battery of loo pairs of plates at the Royal Institution, he found in October 182o that the uniting wire became strongly magnetic and that iron filings clung to it; also that steel needles placed across the wire were permanently magnetized. He placed a sheet of glass over the wire and sprinkling iron filings on it saw that they arranged themselves in straight lines at right angles to the wire. He then proved that Leyden jar discharges could produce the same effects. Ampere and Arago then seem to have experimented together and magnetized a steel needle wrapped in paper which was enclosed in a helical wire conveying a current. All these facts were rendered intelligible when it was seen that a wire when conveying an electric current becomes surrounded by a magnetic field. If the wire is a long straight one, the lines of magnetic force are circular and concentric with centres on the wire axis, and if the wire is bent into a circle the lines of magnetic force are endless loops surrounding and linked with the electric circuit. Since a magnetic pole tends to move along a line of magnetic force it was obvious that it should revolve round a wire conveying a current. To exhibit this fact involved, however, much ingenuity. It was first accomplished by Faraday in October 1821 (Exper. Res. ii. p. 127). Since the action is reciprocal a current free to move tends to revolve round a magnetic pole. The fact is most easily shown by a small piece of apparatus made as follows: In a glass cylinder (see fig. I) like a lamp chimney are fitted two corks. Through the bottom one is passed the north end of a bar magnet which projects up above a little mercury lying in the corks Through the top cork is passed one end of a wire from a armature weighing in all 11.575 lb Joule found that a weight of 2718 was supported. Joule's papers on this subject will be found in his Collected Papers published by the Physical Society of London, and in Sturgeon's Annals of Electricity, 1838–1841, battery, and a piece of wire in the cylinder is flexibly connected to it, the lower end of this last piece just touching the mercury. When a current is passed in at the top wire and out at the lower end of the bar magnet, the loose wire revolves round the magnet pole. All text-books on physics contain in their chapters on electromagnetism full accounts of various forms of this experiment. In 1825 another important step forward was taken when William Sturgeon (1783–1850) of London produced the electromagnet. It consisted of a horseshoe-shaped bar of soft iron, coated with varnish, on which was wrapped a spiral coil of bare copper wire, the turns not touching each other. When a voltaic current was passed through the wire the iron became a powerful magnet, but on severing the connexion with the battery, the soft iron lost immediately nearly all its magnetism.' At that date Ohm had not announced his law of the electric circuit, and it was a matter of some surprise to investigators to find that Sturgeon's electromagnet could not be operated with such good results as when close to the battery. Peter Barlow, in January 1825, published in the Edinburgh Philosophical Journal, a description of such an experiment made with a view of applying Sturgeon's electromagnet to telegraphy, with results which were unfavourable. Sturgeon's experiments, however, stimulated Joseph Henry (q.v.) in the United States, and in 1831 he gave a description of a method of winding electromagnets which at once put a new face upon matters (Silliman's Journal, 1831, 19, p. 400). Instead of insulating the iron core, he wrapped the copper wire round with silk and wound in numerous turns and many layers upon the iron horseshoe in such fashion that the current went round the iron always in the same direction. He then found that such an electromagnet wound with a long fine wire, if worked with a battery consisting of a large number of cells in series, could be operated at a considerable distance, and he thus produced what were called at. that time intensity electromagnets, and which subsequently rendered the electric telegraph a possibility. In fact, Henry established in 1831, in Albany, U.S.A., an electromagnetic telegraph, and in 1835 at Princeton even used an earth return, thereby anticipating the discovery (1838) of C. A. Steinheil (1801–187o) of Munich. Inventors were then incited to construct powerful electro- magnets as tested by the weight they could carry from their armatures. Joseph Henry made a magnet for Yale College, U.S.A., which lifted 3000 lb (Silliman's Journal, 1831, 20, p. 201), and one for Princeton which lifted 3000 with a very small battery. Amongst others J. P. Joule, ever memorable for his investigations on the mechanical equivalent of heat, gave much attention about 1838–184o to the construction of electromagnets and succeeded in devising some forms remarkable for their lifting power. One form was constructed by cutting a thick soft iron tube longitudinally into two equal parts. Insu- lated copper wire was then wound longitudinally over one of both parts (see fig. 2) and a current sent through the wire. In another form two iron disks with teeth at right angles to the disk had insulated wire wound zigzag between the teeth; when a current was sent through the wire, theateeth were so magnetized that they were alternately N. and S. poles. If two such similar disks were placed with teeth of opposite polarity in contact, a very large force was required to detach them, and with a magnet and ' See Trans. Soc.Arts, 1825, 43, p.38, in which a figure of Sturgeon's electromagnet is given as well as of other pieces of apparatus for which the Society granted him a premium and a silver medal.vols. 2-6. The Magnetic Circuit.—The phenomena presented by the electromagnet are interpreted by the aid of the notion of the magnetic circuit. Let us consider a thin circular sectioned ring of iron wire wound over with a solenoid or spiral of insulated copper wire through which a current of electricity can be passed. If the solenoid or wire windings existed alone, a current having a strength A amperes passed through it would create in the interior of the solenoid a magnetic force H, numerically equal to 47r/10 multiplied by the number of windings N on the solenoid, and by the current in amperes A, and divided by the mean length of the solenoid 1, or H =4vrAN/IOl. The product AN is called the " ampere-turns " on the solenoid. The product Hl of the magnetic force H and the length l of the magnetic circuit is called the " magnetomotive force " in the magnetic circuit, and from the above formula it is seen that the magnetomotive force denoted by (M.M.F.) is equal to 47 10(=1.25 nearly) times the ampere-turns (A.N.) on the exciting coil or solenoid. Otherwise (A.N.) =0.8(M.M.F.). The magnetomotive force is regarded as creating an effect called magnetic flux (Z) in the magnetic circuit, just as electromotive force E.M.F. produces electric current (A) in the electric circuit, and as by Ohm's law (see E1.ECTRO1CINETICS) the current varies as the E.M.F. and inversely as a quality of the electric circuit called its " resistance," so in the magnetic circuit the magnetic flux varies as the magnetomotive force and inversely as a quality of the magnetic circuit called its " reluctance." The great difference between the electric circuit and the magnetic circuit lies in the fact that whereas the electric resistance of a solid or liquid conductor is independent of the current and affected only by the temperature, the magnetic reluctance varies with the magnetic flux and cannot be defined except by means of a curve which shows its value for different flux densities. The quotient of the total magnetic flux, Z, in a circuit by the cross section, S, of the circuit is called the mean " flux density," and the reluctance of a magnetic circuit one centimetre long and one square centimetre in cross section is called the " reluctivity " of the material. The relation between reluctivity p=I/p magnetic force H, and flux density B, is defined by the equation H =pB, from which we have Hl=Z(pl/S) = M.M.F. acting on the circuit. Again, since the ampere-turns (AN) on the circuit are equal to o•8 times the M.M.F., we have finally AN/l=o•8(Z/hS). This equation tells us the exciting force reckoned in ampere-turns, AN, which must be put on the ring core to create a total magnetic flux Z in it, the ring core having a mean perimeter 1 and cross section S and reluctivity p=I/p corresponding to a flux density Z/S. Hence before we can make use of the equation for practical purposes we need to possess a curve for the particular material showing us the value of the reluctivity corresponding to various values of the possible flux density. The reciprocal of p is usually called the " permeability " of the material and denoted by p. Curves showing the relation of lip and Z S or p and B, are called " permeability curves." For air and all other non-magnetic matter the permeability has the same value, taken arbitrarily as unity. On the other hand, for iron, nickel and cobalt the permeability may in some cases reach a value of 2000 or 2500 for a value of B = 5000 in C.G.S. measure (see UNITS, PHYSICAL). The process of taking these curves consists in sending a current of known strength through a solenoid of known number of turns wound on a circular iron ring of known dimensions, and observing the time-integral of the secondary current produced in a secondary circuit of known turns and resistance R wound over the iron core N times. The secondary electromotive force is by Faraday's law (see E1.ECTROKiNETics) equal to the time rate of change of the total flux, or E = NdZ/dt. But by Ohm's law E = Rdq/dt, where q is the quantity of electricity set flowing in the secondary circuit by a change dZ in the co-linked total flux. Hence if 2Q represents this total quantity of electricity set flowing in the secondary circuit by suddenly reversing the direction of the magnetic flux Z in the iron core we must have RQ = NZ or Z = RQ/N. The measurement of the total quantity of electricity Q can be made by means of a ballistic galvanometer (q.v.), and the resistance R of the secondary circuit includes that of the coil wound on the iron core and the galvanometer as well. In this manner the value of the total flux Z and therefore of Z/S=B or the flux density, can be found for a given magnetizing force H, and this last quantity is determined when we know the magnetizing current in the solenoid and its turns and dimensions. The curve which delineates the relation of H and B is called the magnetization curve for the material in question. For examples of these curves see MAGNETISM. The fundamental law of the non-homogeneous magnetic circuit traversed by one and the same total magnetic flux Z is that the sum of all the magnetomotive forces acting in the circuit is numerically equal to the product of the factor o•8, the total flux in the circuit, and the sum of all the reluctances of the various parts of the circuit. If then the circuit consists of materials of different permeability and it is desired to know the ampere-turns required to produce a given total of flux round the circuit, we have to calculate from the magnetization curves of the material of each part the necessary magneto-motive forces and. add these forces together. The practical application of this principle to the predetermination of the field windings of dynamo magnets was first made by Drs J. and E. Hopkinson (Phil. Trans., 1886, 177, p. 331). We may illustrate the principles of this predetermination by a simple example. Suppose a ring of iron has a mean diameter of to ems. and a cross section of 2 sq. ems., and a transverse cut on air gap made in it 1 mm. wide. Let us inquire the ampere-turns to be put upon the ring to create in it a total flux of 24,000 C.G.S. units. The total length of the iron part of the circuit is (Iorr-o•1) ems., and its section is 2 sq. ems., and the flux density in it is to be 12,000. From Table II. below we see that the permeability of pure iron corresponding to a flux density of 12,000 is 2760. Hence the reluctance of the iron circuits is equal to 10ir - O. 1 220 2760X2 38640C.G.S. units. The length of the air gap is o•1 cm., its section 2 sq. ems., and its permeability is unity. Hence the reluctance of the, air gap is o X2 =-C.G.S. unit. Accordingly the magnetomotive force in ampere-turns required to produce the required flux is equal to 0.8(24,000) (20+38640) = 1070 nearly. It follows that the part of the magnetomotive force required to overcome the reluctance of the narrow air gap is about nine times that required for the iron alone. In the above example we have for simplicity assumed that the flux in passing across the air gap does not spread out at all. In dealing with electromagnet design in dynamo construction we have, however, to take into consideration the spreading as well as the leakage of flux across the circuit (see DYNAMO). It will be seen, therefore, that in order that we may predict the effect of a certain kind of iron or steel when used as the core of an electromagnet, we must be provided with tables or curves showing the reluctivity or permeability corresponding to various flux densities or-which comes to the same thing-with (B, H) curves for the sample. Iron and Steel for Electromagnetic Machinery.-In connexion with the technical application of electromagnets such as those used in the field magnets of dynamos (q.v.), the testing of different kinds of iron and steel for magnetic permeability has therefore become very important. Various instruments called permeameters and hysteresis meters have been designed for this purpose, but much of the work has been done by means of a ballistic galvanometer and test ring as above described. The "hysteresis" of an iron or steel is that quality of it in virtue of which energy is dissipated as heat when the magnetization is reversed or carried through a cycle (see MAGNETISM), and it is generally measured either in ergs per cubic centimetre of metal per cycle of magnetization, or in watts per lb per 50 or 100 cycles per second at or corresponding to a certain maximum flux density, say 2500 or 600 C.G.S. units. For the details of various forms of permeameter and hysteresis meter technical books must be consulted.' An immense number of observations have been carried out on the magnetic permeability of different kinds of iron and steel, and in the following tables are given some typical results, mostly from experiments made by J. A. Ewing (see Proc. Inst. C.E., 1896, 126, p. 185) in which the ballistic method was employed to determine the flux density corresponding to various magnetizing forces acting upon samples of iron and steel in the form of rings. The figures under heading I. are values given in a paper by A. W.S. Pocklington and F. Lydall (Proc. Roy. Soc., 1892-1893, 52, pp. 164 and 228) as the results of a magnetic test of an exceptionally pure iron supplied for the purpose of experiment by Colonel Dyer, of the Elswick Works. The substances other than iron in this sample were stated to be: carbon, trace; silicon, trace; phosphorus, none; sulphur, 0.013 %; manganese, o•i %. The other five specimens, II. to VI., are samples of commercial iron or steel. No. II. is a sample of Low Moor bar iron forged into a ring, annealed and turned. No. III. is a steel forging furnished by Mr R. Jenkins as a ' See S. P. Thompson, The Electromagnet (London, 1891) ; J. A. Fleming, A Handbook for the Electrical Laboratory and Testing Room, vol. 2 (London, 1903) ; J. A. Ewing, Magnetic Induction in Iron and other Metals (London, 1903, 3rd ed.).sample of forged ingot-metal for dynamo magnets. No. IV. is a steel casting for dynamo magnets, unforged, made by Messrs Edgar Allen & Company by a special pneumatic process under the patents of Mr A. Tropenas. No. V. is also an unforged steel casting for dynamo Magnetiz- a B C.G.S. Units). ing Force Magnetic Flux Density ( )• (C.C.S. Units). I. II. III. IV. V. VI. 5 12,700 10,900 12,300 4,700 9,600 10,900 10 14,980 13,120 14,920 12,250 13,050 13,320 15 15,800 14,010 15,800 14,000 14,600 14,350 20 16,300 14, 580 16,280 15,050 15,310 14,950 30 16,950 15,280 16,810 16,200 16,000 15,66o 40 17,350 15,760 17,190 16,800 16,510 16,150 50 , . 16,060 17,500 17,140 16,900 16,48o 6o . , 16,340 17,750 17,450 17,180 16,78o 70 .. 16, 58o 17,970 17, 750 17,400 17,000 8o .. 16,800 18,18o 18,040 17,620 17,200 90 .. 17,000 18,390 18,230 17,830 17,400 too .. 17,200 18,600 18,420 18,030 17,600 magnets, made by Messrs Samuel Osborne & Company by the Siemens process. No. VI. is also an unforged steel casting for dynamo magnets, made by Messrs Fried. Krupp, of Essen. It will be seen from the figures and the description of the materials that the steel forgings and castings have a remarkably high permeability under small magnetizing force. Table II. shows the magnetic qualities of some of these materials as found by Ewing when tested with small magnetizing forces. Magnetic Flux I. III. VI. Density B. (C.C.S. Units). Pure Iron. Steel Forging. Steel Casting., H µ H u H µ 2,000 0.90 2220 1.38 1450 1.18 1690 4,000 1.40 2850 1.91 2090 I.66 2410 6,000 1.85 3240 2.38 2520 2.15 2790 8,000 2.30 3480 2.92 2740 2.83 2830 10,000 3.10 3220 3.62 2760 4.05 2470 12,000 4.40 2760 4.80 2500 6.65 1810 The numbers I., III. and VI. in the above table refer to the samples mentioned in connexion with Table I. It is a remarkable fact that certain varieties of low carbon steel (commonly called mild steel) have a higher permeability than even annealed Swedish wrought iron under large magnetizing forces. The term steel, however, here used has reference rather to the mode of production than the final chemical nature of the material. In some of the mild-steel castings used for dynamo electromagnets it appears that the total foreign matter, including carbon, manganese and silicon, is not more than 0.3% of the whole, the material being 99.7 % pure iron. This valuable magnetic property of steel capable of being cast is, however, of great utility in modern dynamo building, as it enables field magnets of very high permeability to be constructed, which can be fashioned into shape by casting instead of being built up as formerly out of masses of forged wrought iron. The curves in fig. 3 illustrate the manner in which the flux density or, as it is usually called, the magnetization curve of this mild cast steel crosses that of Swedish wrought iron, and enables us to obtain a higher flux density corresponding to a given magnetizing force with the steel than with the iron. From the same paper by Ewing we extract a number of results relating to permeability tests oft thin sheet iron and sheet steel, such as is used in the construction of dynamo armatures and transformer cores. No.VII. is a specimen of good transformer-plate, 0.301 millimetre thick, rolled from Swedish iron by Messrs Sankey of Bilston. No. VIII. is a specimen of specially thin transformer-plate rolled from scrap iron. No. IX. is a specimen of transformer-plate rolled from ingot-steel. No. X. is a specimen of the wire which was used by J. Swinburne to form the core of his " hedgehog" transformers. Its diameter was o•6o2 millimetre. All these samples were tested in the
End of Article: ELECTROMAGNETISM
[back]
ELECTROLYSIS (formed from Gr. Xbety, to loosen)
[next]
ELECTROMETALLURGY

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.