The magnetic quality of a sample of iron depends very largely upon the purity and physical condition of the metal. The presence of ordinary impurities usually tends to diminish the permeability, though, as will appear later, the addition of small quantities of certain other substances is sometimes advantageous. A very pure form of iron, which from the method of its manufacture is called “steel,” is now extensively used for the construction of dynamo magnets; this metal sometimes contains not more than 0.3% of foreign substances, including carbon, and is magnetically superior to the best commercial wrought iron. The results of some comparative tests published by Ewing (Proc. Inst. C.E., 1896) are given in the accompanying table. Those in the second column are quoted from a paper by F. Lydall and A. W. Pocklington (Proc. Roy. Soc., 1892, 52, 228) and relate to an exceptional specimen containing nearly 99.9% of the pure metal.
| Magnetic Force. | Magnetic Induction. | |||
| Pure Iron. | Low Moor Iron. | Steel Forging. | Steel Casting. | |
| 5 | 12,700 | 10,900 | 12,300 | 9,600 |
| 10 | 14,980 | 13,120 | 14,920 | 13,050 |
| 15 | 15,800 | 14,010 | 15,800 | 14,600 |
| 20 | 16,300 | 14,580 | 16,280 | 15,310 |
| 30 | 16,950 | 15,280 | 16,810 | 16,000 |
| 40 | 17,350 | 15,760 | 17,190 | 16,510 |
| 50 | .. | 16,060 | 17,500 | 16,900 |
| 60 | .. | 16,340 | 17,750 | 17,180 |
| 70 | .. | 16,580 | 17,970 | 17,400 |
| 80 | .. | 16,800 | 18,180 | 17,620 |
| 90 | .. | 17,000 | 18,390 | 17,830 |
| 100 | .. | 17,200 | 18,600 | 18,030 |
To secure the highest possible permeability it is essential that the iron should be softened by careful annealing. When it is mechanically hardened by hammering, rolling or wire-drawing its permeability may be greatly diminished, especially under a moderate magnetizing force. An experiment by Ewing showed that by the operation of stretching an annealed iron wire beyond the limits of elasticity the permeability under a magnetizing force of about 3 units was reduced by as much as 75%. Ewing has also studied the effect of vibration in conferring upon iron an apparent or spurious permeability of high value; this effort also is most conspicuous when the magnetizing force is weak. The permeability of a soft iron wire, which was tapped while subjected to a very small magnetizing force, rose to the enormous value of about 80,000 (Magnetic Induction, § 85). It follows that in testing iron for magnetic quality the greatest care must be exercised to guard the specimen against any accidental vibration.
Low hysteresis is the chief requisite for iron which is to be used for transformer cores, and it does not necessarily accompany high permeability. In response to the demand, manufacturers have succeeded in producing transformer plate in which the loss of energy due to hysteresis is exceedingly small. Tests of a sample supplied by Messrs. Sankey were found by Ewing to give the following results, which, however, are regarded as being unusually favourable. In a valuable collection of magnetic data (Proc. Inst. C.E., cxxvi.) H. F. Parshall quotes tests of six samples of iron, described as of good quality, which showed an average hysteresis loss of 3070 ergs per c.cm. per cycle at an induction of 8000, being 1.6 times the loss shown by Ewing’s specimen at the same induction.
| Limits of Induction. | Ergs per c.cm. per cycle. | Watts per ℔. Frequency, 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 |
The standard induction in reference to determinations of hysteresis is generally taken as 2500, while the loss is expressed in watts per ℔ at a frequency of 100 double reversals, or cycles, per second. In many experiments, however, different inductions and frequencies are employed, and the hysteresis-loss is often expressed as ergs per cubic centimetre per cycle and sometimes as horse-power per ton. In order to save arithmetical labour it is convenient to be provided with conversion factors for reducing variously expressed results to the standard form. The rate at which energy is lost being proportional to the frequency, it is obvious that the loss at frequency 100 may be deduced from that at any other frequency n by simply multiplying by 100/n. Taking the density of iron to be 7.7, the factor for reducing the loss in ergs per c.cm. to watts per ℔ with a frequency of 100 is 0.000589 (Ewing). Since 1 horse-power = 746 watts, and 1 ton = 2240 ℔, the factor for reducing horse-power per ton to watts per ℔ is 746/2240, or just 1/3. The loss for any induction B within the range for which Steinmetz’s law holds may be converted into that for the standard induction 2500 by dividing it by B1.6/25001.6. The values of this ratio for different values of B, as given by Fleming (Phil. Mag., 1897), are contained in the second column of the annexed table. The third column shows the relative amount of hysteresis deduced by Ewing as a general mean from actual tests of many samples (Journ. Inst. Elec. Eng., 1895). Incidentally, these two columns furnish an undesigned test of the accuracy of Steinmetz’s law: the greatest difference is little more than 1%.
| Induction B. | B1.6 25001.6 | Observed relative Hysteresis. |
| 2000 | 0.700 | 0.702 |
| 2500 | 1.000 | 1.000 |
| 3000 | 1.338 | 1.340 |
| 4000 | 2.118 | 2.128 |
| 5000 | 3.031 | 3.000 |
| 6000 | 4.058 | 4.022 |
| 7000 | 5.193 | 5.129 |
| 8000 | 6.430 | 6.384 |
Curves of Permeability and Susceptibility.—The relations of μ (= B/H) to B, and of κ (= I/H) to I may be instructively exhibited by means of curves, a method first employed by H. A. Rowland.[27] The dotted curve for μ and B in fig. 18 is copied from Rowland’s paper. The actual experiment to which it relates was carried only as the point marked X, corresponding to a magnetizing force of 65, and an induction of nearly 17,000. Rowland, believing that the curve would continue to fall in a straight line meeting the horizontal axis, inferred that the induction corresponding to the point B—about 17,500—was the highest that could be produced by any magnetizing force, however great. It has, however, been shown that, if the magnetizing force is carried far enough, the curve always becomes convex to the axis instead of meeting it. The full line shows the result of an experiment in which the magnetizing force was carried up to 585,[28] but though the force was thus increased ninefold, the induction only reached 19,800, and the ultimate value of the permeability was still as much as 33.9.
| Fig. 18. |
| Fig. 19. |