+------+---------+-----------------------------------+
| | | Hysteresis Loss. |
| | +-------------+---------------------+
| Loop.| B (max.)| Ergs per cc.| Watts per lb. per. |
| | | per Cycle. | 100 Cycles per sec. |
+------+---------+-------------+---------------------+
| I. | 1475 | 466 | .300 |
| II. | 2545 | 1,288 | .829 |
| III. | 3865 | 2,997 | 1.934 |
| IV. | 5972 | 7,397 | 4.765 |
| V. | 8930 | 13,423 | 8.658 |
+------+---------+-------------+---------------------+
For most practical purposes the constructor of electromagnetic machinery
requires his iron or steel to have some one of the following
characteristics. If for dynamo or magnet making, it should have the
highest possible permeability at a flux density corresponding to
practically maximum magnetization. If for transformer or
alternating-current magnet building, it should have the smallest
possible hysteresis loss at a maximum flux density of 2500 C.G.S. units
during the cycle. If required for permanent magnet making, it should
have the highest possible coercivity combined with a high retentivity.
Manufacturers of iron and steel are now able to meet these demands in a
very remarkable manner by the commercial production of material of a
quality which at one time would have been considered a scientific
curiosity.
It is usual to specify iron and steel for the first purpose by naming
the minimum permeability it should possess corresponding to a flux
density of 18,000 C.G.S. units; for the second, by stating the
hysteresis loss in watts per lb. per 100 cycles per second,
corresponding to a maximum flux density of 2500 C.G.S. units during the
cycle; and for the third, by mentioning the coercive force required to
reduce to zero magnetization a sample of the metal in the form of a long
bar magnetized to a stated magnetization. In the cyclical reversal of
magnetization of iron we have two modes to consider. In the first case,
which is that of the core of the alternating transformer, the magnetic
force passes through a cycle of values, the iron remaining stationary,
and the direction of the magnetic force being always the same. In the
other case, that of the dynamo armature core, the direction of the
magnetic force in the iron is constantly changing, and at the same time
undergoing a change in magnitude.
It has been shown by F.G. Baily (_Proc. Roy. Soc._, 1896) that if a mass
of laminated iron is rotating in a magnetic field which remains constant
in direction and magnitude in any one experiment, the hysteresis loss
rises to a maximum as the magnitude of the flux density in the iron is
increased and then falls away again to nearly zero value. These
observations have been confirmed by other observers. The question has
been much debated whether the values of the hysteresis loss obtained by
these two different methods are identical for magnetic cycles in which
the flux density reaches the same maximum value. This question is also
connected with another one, namely, whether the hysteresis loss per
cycle is or is not a function of the speed with which the cycle is
traversed. Early experiments by C.P. Steinmetz and others seemed to show
that there was a difference between slow-speed and high-speed hysteresis
cycles, but later experiments by J. Hopkinson and by A. Tanakadate,
though not absolutely exhaustive, tend to prove that up to 400 cycles
per second the hysteresis loss per cycle is practically unchanged.
Experiments made in 1896 by R. Beattie and R.C. Clinker on magnetic
hysteresis in rotating fields were partly directed to determine whether
the hysteresis loss at moderate flux densities, such as are employed in
transformer work, was the same as that found by measurements made with
alternating-current fields on the same iron and steel specimens (see
_The Electrician_, 1896, 37, p. 723). These experiments showed that
over moderate ranges of induction, such as may be expected in
electro-technical work, the hysteresis loss per cycle per cubic
centimetre was practically the same when the iron was tested in an
alternating field with a periodicity of 100, the field remaining
constant in direction, and when the iron was tested in a rotating field
giving the same maximum flux density.
With respect to the variation of hysteresis loss in magnetic cycles
having different maximum values for the flux density, Steinmetz found
that the hysteresis loss (W), as measured by the area of the complete
(B, H) cycle and expressed in ergs per centimetre-cube per cycle, varies
proportionately to a constant called the _hysteretic constant_, and to
the 1.6th power of the maximum flux density (B), or W = [eta]B^(1.6).
The hysteretic constants ([eta]) for various kinds of iron and steel are
given in the table below:--
Metal. Hysteretic Constant.
Swedish wrought iron, well annealed .0010 to .0017
Annealed cast steel of good quality; small
percentage of carbon .0017 to .0029
Cast Siemens-Martin steel .0019 to .0028
Cast ingot-iron .0021 to .0026
Cast steel, with higher percentages of
carbon, or inferior qualities of wrought
iron .0031 to .0054
Steinmetz's law, though not strictly true for very low or very high
maximum flux densities, is yet a convenient empirical rule for obtaining
approximately the hysteresis loss at any one maximum flux density and
knowing it at another, provided these values fall within a range varying
say from 1 to 9000 C.G.S. units. (See MAGNETISM.)
The standard maximum flux density which is adopted in electro-technical
work is 2500, hence in the construction of the cores of
alternating-current electromagnets and transformers iron has to be
employed having a known hysteretic constant at the standard flux
density. It is generally expressed by stating the number of watts per
lb. of metal which would be dissipated for a frequency of 100 cycles,
and a maximum flux density (B max.) during the cycle of 2500. In the
case of good iron or steel for transformer-core making, it should not
exceed 1.25 watt per lb. per 100 cycles per 2500 B (maximum value).
It has been found that if the sheet iron employed for cores of
alternating electromagnets or transformers is heated to a temperature
somewhere in the neighbourhood of 200 deg. C. the hysteresis loss is
very greatly increased. It was noticed in 1894 by G.W. Partridge that
alternating-current transformers which had been in use some time had a
very considerably augmented core loss when compared with their initial
condition. O.T. Blathy and W.M. Mordey in 1895 showed that this
augmentation in hysteresis loss in iron was due to heating. H.F.
Parshall investigated the effect up to moderate temperatures, such as
140 deg. C., and an extensive series of experiments was made in 1898 by
S.R. Roget (_Proc. Roy. Soc._, 1898, 63, p. 258, and 64, p. 150). Roget
found that below 40 deg. C. a rise in temperature did not produce any
augmentation in the hysteresis loss in iron, but if it is heated to
between 40 deg. C. and 135 deg. C. the hysteresis loss increases
continuously with time, and this increase is now called "ageing" of the
iron. It proceeds more slowly as the temperature is higher. If heated to
above 135 deg. C., the hysteresis loss soon attains a maximum, but then
begins to decrease. Certain specimens heated to 160 deg. C. were found
to have their hysteresis loss doubled in a few days. The effect seems to
come to a maximum at about 180 deg. C. or 200 deg. C. Mere lapse of time
does not remove the increase, but if the iron is reannealed the
augmentation in hysteresis disappears. If the iron is heated to a higher
temperature, say between 300 deg. C. and 700 deg. C., Roget found the
initial rise of hysteresis happens more quickly, but that the metal soon
settles down into a state in which the hysteresis loss has a small but
still augmented constant value. The augmentation in value, however,
becomes more nearly zero as the temperature approaches 700 deg. C.
Brands of steel are now obtainable which do not age in this manner, but
these _non-ageing_ varieties of steel have not generally such low
initial hysteresis values as the "Swedish Iron," commonly considered
best for the cores of transformers and alternating-current magnets.
The following conclusions have been reached in the matter:--(1) Iron and
mild steel in the annealed state are more liable to change their
hysteresis value by heating than when in the harder condition; (2) all
changes are removed by re-annealing; (3) the changes thus produced by
heating affect not only the amount of the hysteresis loss, but also the
form of the lower part of the (B, H) curve.
_Forms of Electromagnet._--The form which an electromagnet must take
will greatly depend upon the purposes for which it is to be used. A
design or form of electromagnet which will be very suitable for some
purposes will be useless for others. Supposing it is desired to make an
electromagnet which shall be capable of undergoing very rapid changes of
strength, it must have such a form that the coercivity of the material
is overcome by a self-demagnetizing force. This can be achieved by
making the magnet in the form of a short and stout bar rather than a
long thin one. It has already been explained that the ends or poles of a
polar magnet exert a demagnetizing power upon the mass of the metal in
the interior of the bar. If then the electromagnet has the form of a
long thin bar, the length of which is several hundred times its
diameter, the poles are very far removed from the centre of the bar, and
the demagnetizing action will be very feeble; such a long thin
electromagnet, although made of very soft iron, retains a considerable
amount of magnetism after the magnetizing force is withdrawn. On the
other hand, a very thick bar very quickly demagnetizes itself, because
no part of the metal is far removed from the action of the free poles.
Hence when, as in many telegraphic instruments, a piece of soft iron,
called an armature, has to be attracted to the poles of a
horseshoe-shaped electromagnet, this armature should be prevented from
quite touching the polar surfaces of the magnet. If a soft iron mass
does quite touch the poles, then it completes the magnetic circuit and
abolishes the free poles, and the magnet is to a very large extent
deprived of its self-demagnetizing power. This is the explanation of the
well-known fact that after exciting the electromagnet and then stopping
the current, it still requires a good pull to detach the "keeper"; but
when once the keeper has been detached, the magnetism is found to have
nearly disappeared. An excellent form of electromagnet for the
production of very powerful fields has been designed by H. du Bois (fig.
6).
[Illustration: FIG. 6.--Du Bois's Electromagnet.]
Various forms of electromagnets used in connexion with dynamo machines
are considered in the article DYNAMO, and there is, therefore, no
necessity to refer particularly to the numerous different shapes and
types employed in electrotechnics.
BIBLIOGRAPHY.--For additional information on the above subject the
reader may be referred to the following works and original papers:--
H. du Bois, _The Magnetic Circuit in Theory and Practice_; S.P.
Thompson, _The Electromagnet_; J.A. Fleming, _Magnets and Electric
Currents_; J.A. Ewing, _Magnetic Induction in Iron and other Metals_;
J.A. Fleming, "The Ferromagnetic Properties of Iron and Steel,"
_Proceedings of Sheffield Society of Engineers and Metallurgists_
(Oct. 1897); J.A. Ewing, "The Magnetic Testing of Iron and Steel,"
_Proc. Inst. Civ. Eng._, 1896, 126, p. 185; H.F. Parshall, "The
Magnetic Data of Iron and Steel," _Proc. Inst. Civ. Eng._, 1896, 126,
p. 220; J.A. Ewing, "The Molecular Theory of Induced Magnetism,"
_Phil. Mag._, Sept. 1890; W.M. Mordey, "Slow Changes in the
Permeability of Iron," _Proc. Roy. Soc._ 57, p. 224; J.A. Ewing,
"Magnetism," James Forrest Lecture, _Proc. Inst. Civ. Eng._ 138; S.P.
Thompson, "Electromagnetic Mechanism," _Electrician_, 26, pp. 238,
269, 293; J.A. Ewing, "Experimental Researches in Magnetism," _Phil.
Trans._, 1885, part ii.; Ewing and Klassen, "Magnetic Qualities of
Iron," _Proc. Roy. Soc._, 1893. (J. A. F.)