[1] See Hittorf, _Pogg. Ann._ cvi. 517 (1859).
[2] _Grundriss der Elektrochemie_ (1895), p. 292; see also F. Kaufler
and C. Herzog, _Ber._, 1909, 42, p. 3858.
[3] _Brit. Ass. Rep._, 1906, Section A, Presidential Address.
[4] See _Theory of Solution_, by W.C.D. Whetham (1902), p. 328.
[5] W. Ostwald, _Zeits. physikal. Chemie_, 1892, vol. IX. p. 579; T.
Ewan, _Phil. Mag._ (5), 1892, vol. xxxiii. p. 317; G.D. Liveing,
_Cambridge Phil. Trans._, 1900, vol. xviii. p. 298.
[6] See W.B. Hardy, _Journal of Physiology_, 1899, vol. xxiv. p. 288;
and W.C.D. Whetham, _Phil. Mag._, November 1899.
[7] _Zeits. physikal. Chem._ 2, p. 613.
[8] _Wied. Ann._, 1890, 40, p. 561.
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). 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 therefore 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. Chim._ (1820, 15, p. 94), Arago published a paper entitled
"Experiences relatives a l'aimantation du fer et de l'acier par l'action
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 1820 (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 100 pairs of plates at the Royal
Institution, he found in October 1820 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. 1) 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 cork. Through the top cork is passed one end of a
wire from a 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.
[Illustration: FIG. 1.]
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.[2]
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 at a distance through a
long circuit of wire 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 (1801-1870) of
Munich.
[Illustration: FIG. 2.]
Inventors were then incited to construct powerful electromagnets 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 1838-1840 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. Insulated 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, the teeth 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
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, 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 4[pi]/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 l, or H = 4[pi]AN/10l. 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 4[pi]/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 ELECTROKINETICS) 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
[rho] = 1/[mu] magnetic force H, and flux density B, is defined by the
equation H = [rho]B, from which we have Hl = Z([rho]l/S) = M.M.F.
acting on the circuit. Again, since the ampere-turns (AN) on the
circuit are equal to 0.8 times the M.M.F., we have finally AN/l =
0.8(Z/[mu]S). 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 l and
cross section S and reluctivity [rho] = 1/[mu] 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 [rho]
is usually called the "permeability" of the material and denoted by
[mu]. Curves showing the relation of 1/[rho] and ZS or [mu] 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 ELECTROKINETICS) 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 0.8, 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 magnetomotive 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 10 cms. and a
cross section of 2 sq. cms., 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 (10[pi] - 0.1) cms., and its
section is 2 sq. cms., 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
10[pi] - 0.1 220
------------ = ----- C.G.S. units.
2760 X 2 38640
The length of the air gap is 0.1 cm., its section 2 sq. cms., and its
permeability is unity. Hence the reluctance of the air gap is
0.1 1
----- = -- C.G.S. unit.
1 X 2 20
Accordingly the magnetomotive force in ampere-turns required to
produce the required flux is equal to
/ 1 220 \
0.8(24,000) ( -- + ----- ) = 1070 nearly.
\20 38640/
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.[3]
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, 0.1%. 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 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 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.
TABLE I.--_Magnetic Flux Density corresponding to various Magnetizing
Forces in the case of certain Samples of Iron and Steel_ (_Ewing_).
+------------+-----------------------------------------------------+
|Magnetizing | |
| Force | |
| H (C.G.S. | Magnetic Flux Density B (C.G.S. Units). |
| 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 | 15,660 |
| 40 | 17,350 | 15,760 | 17,190 | 16,800 | 16,510 | 16,150 |
| 50 | .. | 16,060 | 17,500 | 17,140 | 16,900 | 16,480 |
| 60 | .. | 16,340 | 17,750 | 17,450 | 17,180 | 16,780 |
| 70 | .. | 16,580 | 17,970 | 17,750 | 17,400 | 17,000 |
| 80 | .. | 16,800 | 18,180 | 18,040 | 17,620 | 17,200 |
| 90 | .. | 17,000 | 18,390 | 18,230 | 17,830 | 17,400 |
| 100 | .. | 17,200 | 18,600 | 18,420 | 18,030 | 17,600 |
+------------+--------+--------+--------+--------+--------+--------+
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.
TABLE II.--_Magnetic Permeability of Samples of Iron and Steel under
Weak Magnetizing Forces._
+-----------------+-------------+----------------+---------------+
| Magnetic Flux | I. | III. | VI. |
| Density B | Pure Iron. | Steel Forging. | Steel Casting.|
| (C.G.S. Units). | | | |
+-----------------+-------------+----------------+---------------+
| | H [mu] | H [mu] | H [mu] |
| 2,000 | 0.90 2220 | 1.38 1450 | 1.18 1690 |
| 4,000 | 1.40 2850 | 1.91 2090 | 1.66 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 of 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 0.602 millimetre. All these samples were tested in the
form of rings by the ballistic method, the rings of sheet-metal being
stamped or turned in the flat. The wire ring No. X. was coiled and
annealed after coiling.
[Illustration: FIG. 3.]
TABLE III.--_Permeability Tests of Transformer Plate and Wire_.
+---------+--------------+--------------+--------------+--------------+
|Magnetic | VII. | VIII. | IX. | X. |
| Flux | Transformer- | Transformer- | Transformer- | Transformer- |
|Density B| plate of | plate of | plate of | wire. |
| (C.G.S. | Swedish Iron.| Scrap Iron. | of Steel. | |
| Units). | | | | |
+---------+--------------+--------------+--------------+--------------+
| | H [mu] | H [mu] | H [mu] | H [mu] |
| 1,000 | 0.81 1230 | 1.08 920 | 0.60 1470 | 1.71 590 |
| 2,000 | 1.05 1900 | 1.46 1370 | 0.90 2230 | 2.10 950 |
| 3,000 | 1.26 2320 | 1.77 1690 | 1.04 2880 | 2.30 1300 |
| 4,000 | 1.54 2600 | 2.10 1900 | 1.19 3360 | 2.50 1600 |
| 5,000 | 1.82 2750 | 2.53 1980 | 1.38 3620 | 2.70 1850 |
| 6,000 | 2.14 2800 | 3.04 1970 | 1.59 3770 | 2.92 2070 |
| 7,000 | 2.54 2760 | 3.62 1930 | 1.89 3700 | 3.16 2210 |
| 8,000 | 3.09 2590 | 4.37 1830 | 2.25 3600 | 3.43 2330 |
| 9,000 | 3.77 2390 | 5.3 1700 | 2.72 3310 | 3.77 2390 |
| 10,000 | 4.6 2170 | 6.5 1540 | 3.33 3000 | 4.17 2400 |
| 11,000 | 5.7 1930 | 7.9 1390 | 4.15 2650 | 4.70 2340 |
| 12,000 | 7.0 1710 | 9.8 1220 | 5.40 2220 | 5.45 2200 |
| 13,000 | 8.5 1530 | 11.9 1190 | 7.1 1830 | 6.5 2000 |
| 14,000 | 11.0 1270 | 15.0 930 | 10.0 1400 | 8.4 1670 |
| 15,000 | 15.1 990 | 19.5 770 | .. .. | 11.9 1260 |
| 16,000 | 21.4 750 | 27.5 580 | .. .. | 21.0 760 |
+---------+--------------+--------------+--------------+--------------+
Some typical flux-density curves of iron and steel as used in dynamo and
transformer building are given in fig. 4.
[Illustration: FIG. 4.]
The numbers in Table III. well illustrate the fact that the
permeability, [mu] = B/H has a maximum value corresponding to a certain
flux density. The tables are also explanatory of the fact that mild
steel has gradually replaced iron in the manufacture of dynamo
electromagnets and transformer-cores.
Broadly speaking, the materials which are now employed in the
manufacture of the cores of electromagnets for technical purposes of
various kinds may be said to fall into three classes, namely, forgings,
castings and stampings. In some cases the iron or steel core which is to
be magnetized is simply a mass of iron hammered or pressed into shape by
hydraulic pressure; in other cases it has to be fused and cast; and for
certain other purposes it must be rolled first into thin sheets, which
are subsequently stamped out into the required forms.
[Illustration: FIG. 5.]
For particular purposes it is necessary to obtain the highest possible
magnetic permeability corresponding to a high, or the highest attainable
flux density. This is generally the case in the electromagnets which are
employed as the field magnets in dynamo machines. It may generally be
said that whilst the best wrought iron, such as annealed Low Moor or
Swedish iron, is more permeable for low flux densities than steel
castings, the cast steel may surpass the wrought metal for high flux
density. For most electro-technical purposes the best magnetic results
are given by the employment of forged ingot-iron. This material is
probably the most permeable throughout the whole scale of attainable
flux densities. It is slightly superior to wrought iron, and it only
becomes inferior to the highest class of cast steel when the flux
density is pressed above 18,000 C.G.S. units (see fig. 5). For flux
densities above 13,000 the forged ingot-iron has now practically
replaced for electric engineering purposes the Low Moor or Swedish iron.
Owing to the method of its production, it might in truth be called a
soft steel with a very small percentage of combined carbon. The best
description of this material is conveyed by the German term
"Flusseisen," but its nearest British equivalent is "ingot-iron."
Chemically speaking, the material is for all practical purposes very
nearly pure iron. The same may be said of the cast steels now much
employed for the production of dynamo magnet cores. The cast steel which
is in demand for this purpose has a slightly lower permeability than the
ingot-iron for low flux densities, but for flux densities above 16,000
the required result may be more cheaply obtained with a steel casting
than with a forging. When high tensile strength is required in addition
to considerable magnetic permeability, it has been found advantageous to
employ a steel containing 5% of nickel. The rolled sheet iron and sheet
steel which is in request for the construction of magnet cores,
especially those in which the exciting current is an alternating
current, are, generally speaking, produced from Swedish iron. Owing to
the mechanical treatment necessary to reduce the material to a thin
sheet, the permeability at low flux densities is rather higher than,
although at high flux densities it is inferior to, the same iron and
steel when tested in bulk. For most purposes, however, where a laminated
iron magnet core is required, the flux density is not pressed up above
6000 units, and it is then more important to secure small hysteresis
loss than high permeability. The magnetic permeability of cast iron is
much inferior to that of wrought or ingot-iron, or the mild steels taken
at the same flux densities.
The following Table IV. gives the flux density and permeability of a
typical cast iron taken by J.A. Fleming by the ballistic method:--
TABLE IV.--_Magnetic Permeability and Magnetization Curve of Cast
Iron._
+------+------+-----++-------+------+-----++--------+--------+-----+
| H | B | [mu]|| H | B | [mu]|| H | B | [mu]|
| .19 | 27 | 139 || 8.84 | 4030 | 456 || 44.65 | 8,071 | 181 |
| .41 | 62 | 150 || 10.60 | 4491 | 424 || 56.57 | 8,548 | 151 |
| 1.11 | 206 | 176 || 12.33 | 4884 | 396 || 71.98 | 9,097 | 126 |
| 2.53 | 768 | 303 || 13.95 | 5276 | 378 || 88.99 | 9,600 | 108 |
| 3.41 | 1251 | 367 || 15.61 | 5504 | 353 || 106.35 | 10,066 | 95 |
| 4.45 | 1898 | 427 || 18.21 | 5829 | 320 || 120.60 | 10,375 | 86 |
| 5.67 | 2589 | 456 || 26.37 | 6814 | 258 || 140.37 | 10,725 | 76 |
| 7.16 | 3350 | 468 || 36.54 | 7580 | 207 || 152.73 | 10,985 | 72 |
+------+------+-----++-------+------+-----++--------+--------+-----+
The metal of which the tests are given in Table IV. contained 2% of
silicon, 2.85% of total carbon, and 0.5% of manganese. It will be seen
that a magnetizing force of about 5 C.G.S. units is sufficient to impart
to a wrought-iron ring a flux density of 18,000 C.G.S. units, but the
same force hardly produces more than one-tenth of this flux density in
cast iron.
The testing of sheet iron and steel for magnetic hysteresis loss has
developed into an important factory process, giving as it does a means
of ascertaining the suitability of the metal for use in the manufacture
of transformers and cores of alternating-current electromagnets.
In Table V. are given the results of hysteresis tests by Ewing on
samples of commercial sheet iron and steel. The numbers VII., VIII., IX.
and X. refer to the same samples as those for which permeability results
are given in Table III.