This page has been proofread, but needs to be validated.
230
ELECTROMAGNETISM

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 μ H B μ H B μ
.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.

Table V.Hysteresis Loss in Transformer-iron.

Maximum
Flux
Density
B.
Ergs per Cubic Centimetre
per Cycle.
Watts per ℔ at a Frequency
of 100.
VII.

Swedish
Iron.
VIII.
Forged
Scrap-
iron.
IX.

Ingot-
steel.
X.
Soft
Iron
Wire.
VII. VIII. IX. X.
2000 240 400 215 600 0.141 0.236 0.127 0.356
3000 520 790 430 1150 0.306 0.465 0.253 0.630
4000 830 1220 700 1780 0.490 0.720 0.410 1.050
5000 1190 1710 1000 2640 0.700 1.010 0.590 1.550
6000 1600 2260 1350 3360 0.940 1.330 0.790 1.980
7000 2020 2940 1730 4300 1.200 1.730 1.020 2.530
8000 2510 3710 2150 5300 1.480 2.180 1.270 3.120
9000 3050 4560 2620 6380 1.800 2.680 1.540 3.750

In Table VI. are given the results of a magnetic test of some exceedingly good transformer-sheet rolled from Swedish iron.

Table VI.Hysteresis Loss in Strip of Transformer-plate rolled
Swedish Iron.

Maximum Flux
Density B.
Ergs per Cubic Centimetre
per Cycle.
Watts per ℔ at a
Frequency of 100.
2000  220 0.129
3000  410 0.242
4000  640 0.376
5000  910 0.535
6000 1200 0.710
7000 1520 0.890
8000 1900 1.120
9000 2310 1.360

In Table VII. are given some values obtained by Fleming for the hysteresis loss in the sample of cast iron, the permeability test of which is recorded in Table IV.

Table VII.Observations on the Magnetic Hysteresis of Cast Iron.

Loop. B (max.) Hysteresis Loss.
Ergs per cc.
per Cycle.
Watts per ℔ per.
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 ℔ 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. Tanakadaté, 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