ELECTROMAGNET. Soon after the discovery, by Oersted in 1820, of the directive action of an electric current on a magnetic needle, it was found that a wire carrying an electric current also possessed the power of magnetizing pieces of steel and iron placed near it. This discovery was made by D. F. J. Arago (Ann. de Chim. et de Physique, 1820), who announced that a copper wire conveying an electric current could magnetize steel needles placed across it, and could attract iron filings. In October of the same year these facts were discovered inde pendently by Sir Humphry Davy (Phil. Trans. R. S., 1820). In one of Davy's experiments a number of steel needles were fastened to a silver wire in different directions, some being parallel and others transverse to the wire. When the ends of the wire were connected to a battery the needles parallel to the wire attracted iron filings in much the same way as did the wire itself, and they lost this power when the connection with the battery was broken. Each of the needles placed across the wire, however, acquired two magnetic poles, and retained its magnetism after the current was broken. Pieces of iron wire placed across the silver wire also became magnetized, and they lost their magnetism when the battery connection was broken. About the same time Arago and A. M. Ampere magnetized a steel needle which was wrapped in paper and placed within a helical coil of wire carry ing a current.
It was a short but extremely important step from these pre liminary discoveries to the production of the electromagnet, a combination of coil of wire and iron core which was destined to become one of the principal elements in the successful develop ment of the technical applications of electricity. The step was taken by William Sturgeon, a native of Whittingham, in Lanca shire, where he was born in 1783. • He was a shoemaker by trade and for 18 years a private soldier in the Royal Artillery, entirely self-taught, but an enthusiastic experimenter in electricity. Stur geon's electromagnet, front and side views of which are shown in fig. I, was described by him in a communication to the Society of Arts in 1825 (Trans. Society of Arts, 1825). It consisted of a bent bar of soft iron, about a foot long and half an inch in diameter, coated with varnish, on which was wound a coil of bare copper wire of 18 turns. The ends of the wire dipped into mercury cups, mounted on a wood stand, for leading in the electric current. When a current was passed through the wire the bar became a strong magnet, and could support a heavy iron mass applied to its ends, as at y, fig. 1. When the connection at d was broken, the weight immediately fell. The weight of the bent iron core of Sturgeon's electromagnet was 7 oz., and when excited by the current from a single cell it supported a weight of 9 lb. Sturgeon's experiments soon attracted the attention of other investi gators, among them Joseph Henry in the United States, who in 1831 described an important improvement which he had effected (Silliman's Journal, 1831). In stead of insulating the iron bar he covered the wire with silk, and wound a great number of turns of it round the bar in the same direction. A horseshoe shaped electromagnet which he made in this way for Princeton lifted 75o lb.
when excited by the current from a small battery, and another one, for Yale college, supported 2,086 lb. Henry also found that his electromagnets could be op erated by a battery at a considerable distance, and actually succeed by this means in establishing an electromagnetic telegraph in 1831.
In 1838 Sturgeon removed to Manchester, where he came into contact with J. P. Joule, who afterwards became famed for his researches on the equivalence of heat and mechanical work. During the two following years Joule devoted much attention to the study of the lifting power of electromagnets. One form which he constructed consisted of a thick-walled tube of wrought iron, cut longitudinally into two unequal parts. The larger part was wound longitudinally with insulated copper wire, and the smaller part acted as keeper. This electromagnet, the iron of which weighed 15 lb., was found to be capable of supporting a weight of 2,090 lb. In another form of lifting magnet designed by Joule, a number of flat pieces of iron were bolted to one side of a strong brass ring, and had rectangular grooves cut in them of width equal to the spaces between them. A bundle of insulated copper wires was bent to and fro in the grooves and the spaces. A similar set of pieces of iron, but without grooves, was bolted to another brass ring, and applied to the first set, as armature, so that the grooves in the first covered the spaces in the second. A "zigzag ring of iron" was thus formed, and with the current from 16 cells a weight of 2,710 lb. was supported, the weight of the iron pieces being in all 11•575 lb. (See Joule's Scientific Pa pers published by the Physical Society of London.) Lifting magnets are now much used for lifting steel ingots, tubes, girders, scrap iron, etc., and for handling metal when it is at a high temperature. They are also made watertight so that they can operate under water. Modern lifting magnets are usually of the bell or mushroom form, the magnetizing coil being placed round the central core, and the poles being therefore at the centre and the rim. In fig. 2 is shown a lifting magnet in operation, and the steam engine and dynamo for generating the current supplied to the magnet. A magnet of this type, 66in. in diameter, and weighing nearly 3 tons, can support ten times its weight of iron.
Owing to the great normal pressure between strongly mag netized surfaces in contact, a large amount of friction may be called into play, and this force, which is easily controllable, is put to practical use in electromagnetic clutches, chucks and brakes. Electromagnets are also used, for a great variety of pur poses, in producing movement at a distance from the operator, as in electric bells, telephone receivers and wireless loud speakers. The electromagnet provides, in fact, by far the most convenient means of producing such effects. Probably the most important practical applications of the electromagnet, however, are found in the dynamo and the electric motor, where it is used for pro ducing the electric field in which the armature rotates, and in the transformer, where a varying field is caused to induce current in a secondary circuit.
In addition to its practical uses, the electromagnet has also played a part in several important scientific discoveries. It was used by Faraday in his discovery of the rotation of the plane of polarization of light in a magnetic field, and other investigations in magnetism. One of Faraday's electromagnets is still preserved at the Royal Institution. The Kerr magnetic effect, a rotation of the plane of polarization of light reflected from the polished pole piece of a magnet, was discovered by Dr. John Kerr with an elec tromagnet now in the Natural Philosophy department of the Uni versity of Glasgow. An electromagnet was also used by Zeeman in his great discovery of the effect now known by his name, a splitting of the spectral lines of a source of light placed in a magnetic field. The practical and scientific applications of elec tromagnets are almost innumerable. All we can here attempt is to make a broad classification of them, and to give an outline of the principles underlying the action of electromagnets, and the design of electromagnets for the various purposes which they are intended to serve.
The Magnetic Circuit.—Every electromagnet includes an electric circuit linked with a magnetic circuit in which there is a certain amount of magnetic flux, or number of tubes of magnetic induction. The relation between the electric current and the mag netic flux is expressed by the equation of the magnetic circuit, which is based upon the two fundamental principles :—(1) The line integral of the magnetic force H taken round any closed curve, known as the magnetomotive force in the curve, is equal to 4 irni, where n is the number of turns of wire linked with the curve, and i is the current in the wire in c.g.s. units. (2) Just as the electric current is the same in all parts of the electric circuit, so the total magnetic flux is the same in all parts of the magnetic circuit.
The relation between the magnetomotive force and the flux in a magnetic circuit is expressed in terms of a quantity known as the magnetic reluctance of the circuit. This quantity depends upon the dimensions of the circuit and upon the magnetic quality of the terials of which it is composed. Its value may be derived in the following manner. In any portion of the circuit of mean length 1 and cross section S, let the mean value of the magnetic force be H, and let N denote the total flux. Then the mean value of the flux density, or magnetic induction, B, is N/S, and from the tion of magnetic permeability µ H = B/µ= N/p.S, and therefore Hl = Nl/pS. Forming similar equations for the other portions of the circuit and adding them together, bearing in mind that N is the same in all parts of the cuit, we have, on the left-hand side of the resulting equation, the magnetomotive force in the circuit, and, on the right hand side, the product of the flux and the sum of the quotients l/j.S for all parts of the circuit. This sum is what is known as the netic reluctance of the circuit. It may also be expressed as the sum of the quantities 1p/S for all parts of the circuit, where p is the reluctivity of the material, i.e., the reciprocal of the permeability. If the current is expressed in amperes, the magnetomotive force is 4 lrni/IO, or 1•2S7ni. Thus the equation of the magnetic circuit is I•257Xampere-turns (ni)=NXmagnetic reluctance. This equation is analogous to that which expresses Ohm's law for an electric circuit ; viz., electromotive force = currentXresistance. An important difference between the electric circuit and the mag netic circuit arises from the fact that, whereas the electric resist ance of a solid or liquid conductor is independent of the current so long as the temperature and other physical conditions are con stant, the magnetic reluctance varies with the magnetic flux, and for its determination a knowledge of the permeability of the materials at different flux densities is required. From the perme ability curves of the materials the magnetic reluctance of each portion of the magnetic circuit can be calculated, and hence the ampere-turns required to produce a given flux in the circuit.
As an example, suppose that a ring of iron has a mean diameter of Iocm. and a cross section of 2sq.cm., and a transverse cut or air gap made in it Imm. wide. Let us inquire the ampere-turns required on the ring to create in it a total flux of 24,000 c.g.s. The length of the iron part of the circuit is 'or — o• 1 cm., and as its section is 2sq.cm., the flux density in it is 12,000 c.g.s. units. Assuming that the permeability of pure iron at this flux density is 2,76o, we find that the reluctance of the iron portion of the circuit is ( o•I)/2X2,76o, or about 0.0057 c.g.s. unit. The length of the air gap is 0.1 cm., its section 2sq.cm., and its permeability unity. Hence the reluctance of the air gap is o• I/2 = 0.05 c.g.s. Consequently the number of ampere-turns required to produce the flux is ni— 24,000 0.00 0•0 106o (nearly).
The metal of which the tests are given in Table II. contained 2% of silicon, 2.85% of total carbon, and 0.5% of manganese. In fig. 3 the flux density and the permeability for other typical samples of commercial iron and steel for electromagnets (from data in Miles Walker, Specification and Design of Dynamo Elec tric Machinery, 1918), are shown in the form of curves. Curve I. is the magnetization curve, or (B, H) curve of a sample of cast iron. Curve II. is the magnetization curve of an annealed steel casting, unforged, containing 0.2 % of carbon. Curve II.a shows the permeability (upper horizontal scale) of this material for various values of B shown on the vertical scale. Curves III. and III.a are the magnetization and permeability curves for an annealed sample of forged ingot iron. Curves IV. and IV.a similarly show the magnetic properties of a sample of silicon steel containing 4.8% of silicon and 0.2% of carbon. Ingot iron is probably the purest iron which can be obtained commercially in large quantities. The sample the magnetic properties of which are shown in curves III. and III.a, fig. 3, contained 0.15% of carbon, but in some samples the total foreign matter, including carbon, manganese, sulphur, does not amount to this percentage of the whole. Owing to its relatively high permeability at all values of the flux density, this material is very suitable for the cores of dynamo magnets. It is, however, not very suitable for electromagnets supplied with alternating currents, owing to its low specific resistance, or resistivity, and the consequent losses in it due to eddy currents. The maximum permeability of the specimen represented in curves III. and III.a, fig. 3, is 2,320 at the flux density B=io,000 c.g.s.
Magnet cores of cast steel (mild steel) are less costly than those of forged metal, and, as shown in the foregoing tables and curves, mild steel is little inferior to the latter in its magnetic quality. Steel castings are therefore much used for the cores and yokes of dynamo magnets. As indicated in Table II. and curve I, fig. 3, the magnetic properties of cast iron are much inferior to those of the low-carbon steels. Cast iron is, however, used for the yokes and frames of dynamo machines, being easily cast and inexpensive, but it is less used now than it was formerly.
It was discovered in 1903 by Sir Robert Hadfield that certain iron-silicon alloys have very high permeability at low values of the flux density. This property is illustrated by curves IV. and IV.a in fig 3, the maximum permeability for the specimen repre sented being 3,790 at the induction 8,000 c.g.s. In stronger fields, however, the presence of the silicon lowers the permeability.
Silicon steel is not suitable for the cores of electromagnets in which high flux density is re quired, but, owing to the compar atively small dissipation of ener gy in this material when the flux is alternating, it is largely used in the form of thin sheet for the cores of alternate current trans formers. The best qualities of silicon steel contain 3.5 to 4.5% of silicon, and practically no other foreign matter. With much larger proportions of silicon the material tends to become brittle, and the permeability in strong fields is further dimin ished. With smaller proportions of silicon this material is also used for the armatures of dyna mos, but for this purpose stampings of dynamo sheet-steel, con taining less than 1% of silicon are more suitable, owing to the higher permeability of this material at high flux densities.
When sheet-steel is used in electromagnets it should be placed so that as far as possible the direction of magnetization is parallel to the grain, i.e., the direction in which the sheet has been rolled.
The magnetic properties in a direction at right-angles to the grain are usually inferior to those in the grain direction. The maximum permeability of a material may be considerably increased by careful re-annealing. Values as high as 12,000 have been found by Gumlich in iron-silicon alloys as the result of this treatment. The effect of preparing and annealing iron and iron-silicon alloys in vacuo has been studied by Yensen (Proc. Am. I.E.E., 1915, 1916), who found great increase in the maximum permeability, the effect being due to a reduction in the impurities, especially carbon.
The remarkable magnetic properties of two other alloys may be referred to briefly. The material known as "permalloy," a very pure nickel-iron alloy containing about 78.5% of nickel, shows very high permeability in weak magnetizing fields. Under a mag netic force of c.g.s. unit the induction in permalloy may be as great as 5,000 c.g.s units the permeability being therefore 100,000. If a straight copper wire of diameter imm., carrying a current of 1/800 ampere, were closely surrounded by a sheath of permalloy, the induction in the innermost layer of the sheath might be about 5,000 c.g.s. The self-inductance of the wire would thus be greatly increased. It is for "loading" a wire in this way that permalloy is at present chiefly useful in connection with telegraph and telephone cables. In strong fields the permea bility of permalloy is inferior to that of good cast iron. A very different property is possessed by the cobalt-iron alloy containing about 34.5% of cobalt. This alloy was shown by P. Weiss to have remarkably high magnetization in strong fields, the saturation induction B–H being about 12% higher than that of pure iron. This material is very suitable for the pole-pieces of electromag nets used in producing very intense magnetic fields.
Much attention has been devoted to the study of the hysteresis loss as affected by the maximum flux density in the cycle and by the nature and state of the mate rial, and various practical meth ods of measuring hysteresis have been devised. The results may be expressed in ergs per cycle per cubic centimetre of the material, or in watts per pound at some stated frequency of alternation and at some maximum flux den sity, say, at 5o cycles per second and io,000 c.g.s units of flux density. The relative qualities in regard to hysteresis of two kinds of sheet-steel are illustrated in fig.
4, which shows the upper portions of their hysteresis loops as deter mined by H. Hoffmann (Arch. f. Elektrot., 1913). The full line curve refers to an annealed specimen of silicon steel, the broken line curve to a sample of ordinary sheet steel. The superiority of the silicon steel is shown by the relative smallness of the area of its hysteresis loop, and the consequent low value of the hys teresis loss in this material. Silicon steel of good quality is also practically free from the defect known as "ageing," i.e., the increase in the hysteresis loss which occurs in ordinary iron or steel when subjected for prolonged periods to temperatures in the neighbourhood of 100° C.
Various empirical formulae have been proposed to express the hysteresis loss in terms of the maximum flux density in the cycle. The best known of these is that of Steinmetz, which represents the hysteresis loss by the expression hB's, h being a constant. The value of h is very small in the iron-silicon alloys (see MAGNET ISM). For the specimen represented by the full-line curve in fig. 4 the value of h was o•ooi, when the hysteresis loss is expressed in ergs per cycle per cubic centimetre. For the sample of ordinary sheet-steel represented in fig. 4 the value of h was 0.0024. In a very pure annealed specimen of alloyed steel o•5mm. thick, con taining 4.09% of silicon and 0.07% of carbon, Gumlich found the value o.0006 for h.
The other core loss in alternating current cores (often more than one half of the total core loss) arises from the eddy currents induced in the cores by the changes of flux, and it is for the pur pose of reducing this loss that the cores of alternating current electromagnets are made of thin sheets of material, insulated from one another, and so disposed that their planes are parallel to the direction of the flux. The theory of eddy-currents in lami nated cores was given by J. J. Thomson (Electrician, 1892), who showed that the energy dissipated owing to this cause per second per cubic centimetre of the material is proportional, when the quantity ird J/ is small, to where d is the thick ness of each sheet, n is the frequency of alternation, B the maxi mum induction, and a the specific resistance of the material. In low-frequency alternations therefore the eddy-current loss dimin ishes as the specific resistance of the material increases. The spe cific resistance of alloyed steel may be five or six times as great as that of ordinary steel sheet, and the eddy-current loss is there fore smaller in the former material. This is another reason why silicon steel is a superior material to ordinary sheet-steel for transformer and other alternating current cores. (See TRANS FORMERS.) It will be noted that the eddy-current loss per second is proportional to the square of the frequency, and this fact forms the basis of one of the chief methods of separating the eddy-current loss from the hysteresis loss, the latter being pro portional to the frequency. In commercial silicon steel plates of good quality o• 5mm. thick, at a maximum induction of i o,000 c.g.s. units and frequency 5o per second, the combined hysteresis and eddy-current losses amount to less than 1 watt per pound of the material.
Electromagnets for Producing Intense Magnetic Fields. —The problem of producing a very intense magnetic field in a small air gap in a magnetic circuit is very different from that of producing a large amount of total flux in the armature of a dynamo. In the latter case great concentration of the flux is not desired. The flux density in the air gaps separating the pole pieces from the armature of a dynamo does not usually exceed about i 5,000c.g.s. units, but in electromagnets used for the pur pose of examining the magnetic properties of iron and other mag netic materials in very strong fields, and for many physical ex periments, a much greater degree of concentration of the flux is required.
The magnetic field in the air gap may be regarded as made up of two parts ; viz., that due to the magnetic poles on the sides of the gap, and that due directly to the magnetic action of the current in the magnetizing coils. Unless this current is extremely strong the magnetic field which it produces in the gap is very small in comparison with that due to the magnetism on the pole faces. We conclude, therefore, that, if the pole-faces are parallel planes extending across the whole cross section of the core, the magnetic field in the gap between them cannot much exceed the saturation flux density of the iron, which is about 21,600 c.g.s. units. It is clear that, in the case of the gap with plane parallel sides, the poles near the periphery of the gap do not contribute much to the magnetic field at the centre. They might be expected to contribute more if they were laid back, away from the gap, so that they could produce greater magnetic force in the axial direction at the centre of the gap. This is found to be the case experimentally, and conical pole-pieces, having surfaces in the form of truncated cones, are generally used in electromagnets for producing very intense fields. The angles which the cones should have in order to produce the most concentrated field was calculated by Stefan and by Ewing in 1888 on the assumption that the pole-pieces are uniformly magnetized to saturation in the axial direction. The calculation showed that the pole-faces should take the form of cones of semi-vertical angle 44', and that with this angle the value of the field at the vertex of the cones is given by the expression o.886(B–H) where b is a the radius of the base of the cones, and a is the radius of the cone at the narrow end, i.e., the radius of the gap. Assuming the saturation value 21,60o for B–H this gives 19,140 b as the a maximum field strength produced by conical pole-faces. To this must be added the field due to the magnetism on the narrow plane ends of the cones (unless the gap is bridged across by a narrow neck or "isthmus"), and the field due to the magnet coils. In practice the angle of the cores for maximum concentra tion should, as pointed out by Ewing, be rather greater than 44', owing to the fact that the magnetization of the pole pieces is not quite uniform. The question of the best form of pole-piece was examined experimentally by du Bois, who found that for maximum concentration the angle should also increase slightly towards the base of the cone. The coils should be placed on the electromagnet so as to produce, by the direct action of the current, the greatest possible field in the gap, that is, their end windings should be as near the gap as possible. In this position the coils will also have their greatest effect in saturating the pole-pieces.
In 1891 du Bois designed a large electromagnet in which the core was a ring of Swedish iron of mean diameter 5ocm. and diameter of section iocm. The coils contained 2,400 turns of wire which could carry 5o amperes. The weight of this electromagnet was about scwt. With conical pole-pieces the electromagnet gave a maximum field of 4o,000c.g.s. units (or gauss) in a gap imm. wide and 6mm. in diameter. Subsequently du Bois designed a more convenient form of electromagnet, known as the half-ring type, which is now much used in experimental and testing work. The latest type of du Bois half-ring electromagnet is illustrated in fig. 5 (from the Zeitschri f t fur Instrumentenkunde, 1911), where it is shown mounted on a turn-table so that it can be rotated easily about a vertical axis. In this type each of the two curved cores of cast-steel forms about one-third of a complete ring, the diameter of the core section increasing towards the base. The form of the coils is such as to allow optical or other apparatus to be brought close up to the borings in the cores, and extra polar coils are provided which can be slipped over the pole pieces so as to be in the most effective position for increasing the field. A copper tube also surrounds the upper end of each core, to carry a current of water for cooling. This electromagnet is made in several sizes. In one model, weighing 7cwt., the maxi mum field in a gap imm. wide and 6mm. diameter is 50,00o gauss. With a smaller gap of o.5X3mm. and with ferro-cobalt pole-tips the field is S9,000 gauss. A larger model weighing 27cwt. and having cores aocm. in diameter at the upper end, gives in the o•5X3mm. gap a field of 65,00o gauss.
Very powerful electromagnets have also been designed by P. Weiss Clown. de Phys., 1907) who has adopted and improved upon the well known Ruhmkorff pattern. In this type the coils are carried by two horizontal coaxial cores supported by a massive yoke. In the Weiss electromagnets very adequate arrangements are made for cooling the coils, one method being to immerse the windings in oil cooled by water circulating in a spiral tube. A magnet of this type weighing 2 51cwt., excited by a current of 6o amperes, gave a field of 46,00o gauss in a 2X6mm. gap. In a later and still larger model (Arch. des Sciences, 1917) the cur rent of ioo amperes is carried by 1,440 turns of copper tubing which also conveys the current of cooling water. The ten sec tions into which the coils are divided are connected in series for the electric current, and in parallel for the current of cooling water. A very large electromagnet of this type has recently been completed at Paris, where it is installed at the Office of Research and Inventions. It is said to have cores over one metre in diameter at the base, to be wound with 5,00o metres of copper tube and to weigh over i oo tons.
The general effect of increase of dimensions of an electro magnet on the field which it produces may be gathered from the principle of similarity, stated by Lord Kelvin, which may be ex pressed as follows :—If the linear dimensions of an electromagnet are increased in any ratio, and if the current in the coils is in creased in the same ratio, the flux density at corresponding points will be unaltered. If, for example, the linear dimensions of an electromagnet were all doubled, and the current also doubled, the field intensity in the gap would be the same as before. The linear dimensions of the gap between the pole-tips would, how ever, be twice as great as before, and if the gap were reduced to its former size there would be an increase of field intensity equal to about 19,140 that is about 5,740 gauss, assuming the pole-pieces to be saturated. It is clear therefore that no very great increment of field intensity can be expected as the result of any reasonable increase in the dimensions of a large electromagnet of the ordinary type, in which the field is mainly due to the magnetism of the pole-pieces. There remains, however, the pos sibility of increasing the current in the magnet coils so greatly that the field due directly to the current becomes a large frac tion of the total field.
This procedure was adopted in 1914 by Deslandres and Perot (Comptes Rendus, 1914) who, with a current of 5,00o amperes flowing in a water-cooled spiral of silver ribbon, and without an iron core, produced a field intensity of 49,90o gauss. When the spiral was provided with an iron core a field intensity of 63,700 c.g.s. units was attained. The method of producing intense mag netic fields by means of very strong currents has been much de veloped, at Cambridge, by P. Kapitza (Proc. Roy. Soc., A, 1924)• In his earlier experiments Kapitza, using specially constructed accumulator batteries and switch gear, passed currents up to 8,000 amperes (measured by a shunted oscillograph) through a coil for short intervals of time. In a coil of imm. internal diam eter, fields of the order soo,000 gauss for 0.003 second were ob tained in this way. In his later experiments (ibid., 1927) still stronger currents were produced by short-circuiting an alternating current generator of special construction, and a field of 320,000 gauss in a volume of 2cu.cm. was obtained for .oi second.
Mechanical Forces Produced by Electromagnets.—Elec tromagnets are used in different ways for exerting mechanical force on bodies placed near them. The force causing a bar of iron to adhere to the poles of a horseshoe magnet, known as the tractive force of the magnet, fol lows a different law from that of the force acting on a small mag netic body placed in the field of a magnet. In another class of applications, the most important example of which is found in the electric motor (see MOTORS,