| Saturation Value of I | |
| Wrought iron | 1,700 |
| Cast iron | 1,240 |
| Nickel (0.75% iron) | 515 |
| Nickel (0.56% iron) | 400 |
| Cobalt (1.66% iron) | 1,300 |
It is shown in the paper that the greatest possible force which the isthmus method can apply at a point in the axis of the bobbin is
F = 11.137 Is log10 b/a,
Is being the saturation value of the magnet poles, a the radius of the neck on which the cones converge, and b the radius of the bases of the cones.
Some experiments made by H. du Bois (Phil. Mag., 1890, 29, 293) with an electromagnet specially designed for the production of strong fields, confirm Ewing’s results for iron, nickel and cobalt. The method employed did not admit of the production of such high magnetizing forces, but was of special interest in that both B and I were measured optically—B by means of the rotation of a polarized ray inside a glass plate, as before described, and I by the rotation of a polarized ray reflected from the polished surface of the magnetized metal (see “Kerr’s constant,” [Magneto-Optics]). H(= B − 4πI) was calculated from corresponding values of I and B. Taylor Jones (Wied. Ann., 1896, 57, 258, and Phil. Mag., 1896, 41, 153), working with du Bois’s electromagnet and using a modification of the isthmus method, succeeded in pushing the induction B up to 74,200 with H = 51,600, the corresponding value of I being 1798, and of μ only 1.44. The diameter of the isthmus was 0.241 mm., and the electromagnet was excited by a current of 40 amperes.
Tractive Force of a Magnet.—Closely connected with the results just discussed is the question what is the greatest tractive force that can be exerted by a magnet. In the year 1852 J. P. Joule (Phil. Mag., 1852, 3, 32) expressed the opinion that no “force of current could give an attraction equal to 200 ℔ per sq. in.,” or 14,000 grms. per square centimetre, and a similar view prevailed among high authorities more than twenty years later. For the greatest possible “lifting power” of permanent magnets this estimate is probably not very far from the truth, but it is now clearly understood that the force which can be exerted by an electromagnet, or by a pair of electromagnets with opposite poles in contact, is only limited by the greatest value to which it is practically possible to raise the magnetizing force H. This is at once evident when the tractive force due to magnetization is expressed as 2πI² + HI. For fields of moderate intensity the first term of the expression is the more important, but when the value of H exceeds 12,000 or thereabouts, the second preponderates, and with the highest values that have been actually obtained, HI is several times greater than 2πI². If H could be increased without limit, so also could the tractive force. The following table shows the greatest “lifting powers” experimentally reached at the dates mentioned:—
| Observer. | Kilos per sq. cm. | ℔ per sq. in. | Date. |
| Joule | 12.3 | 175 | 1852 |
| Bidwell | 15.9 | 226 | 1886 |
| Wilde | 26.8 | 381 | 1891 |
| T. Jones | 114.9 | 1634 | 1896 |
5. Magnetization in Very Weak Fields
Some interesting observations have been made of the effects produced by very small magnetic forces. It was first pointed out by C. Baur (Wied. Ann., 1880, 11, 399) that in weak fields the relation of the magnetization I to the magnetizing force H is approximately expressed by an equation of the form
I = aH + bH²,