| Magnetizing Force H (C.G.S. Units). | Magnetic Flux Density B (C.G.S. 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 Density B (C.G.S. Units). | I. Pure Iron. | III. Steel Forging. | VI. Steel Casting. | |||
| H | μ | H | μ | H | μ | |
| 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.
| Fig. 3. |