256 M A G N E T I S M magnetization, and so on ; the results for cobalt are, how ever, held to be less satisfactory than those for iron and nickel, for a variety of reasons which he assigns. In treating his results graphically, two methods are followed. In the first the magnetic induction |8 is plotted against the magnetizing force |p as abscissa. Figure 34 shows the curve obtained in this way from one of his tables. In the second method (1) the permeability za- is plotted aga-inst the magnetic induction |3, or (2) the susceptibility K is plotted against the intensity of magnet ization JI. Either variety of the second method leads to a curve having the general form shown in figures 35 and 36. 5,000 10.000 15,000 Vic-. 35. Curve ~r*3 for Iron. FIG. 36. Curves zsr|3 for Nickel at different temperatures. The curves obtained, whether for w and $, or for K and |[, fall very rapidly, and ultimately to all appearance almost straight, towards the axis of ^ or $. This suggests that 28 or | or both reach a maximum when -p is increased indefinitely. Supposing such an increase of p possible, the question arises as to which it is that actually reaches a maximum. Most experimenters seem to assume that ^ does so, but it must be remarked that this is simply an assumption. 1 Several delicate points of great physical interest might be discussed here, but it will be sufficient to refer the reader to the introduction to Rowland s second paper. The general conclusions to be drawn from these experi ments are as follows : 1. The magnetic properties of iron, nickel, and cobalt at ordinary temperatures differ in degree but not in quality. 2. As the magnetizing force |) increases from upwards, the permeability of iron, nickel, and cobalt increases until 1 All the more so that it has been found by some experimenters that the curve (K|) actually has a point of inflexion and becomes convex to the axis of | for very large values of $ij. it reaches a maximum, and. after that diminishes down to a very small value. The maximum value 2 is reached when the metal has attained a magnetization of from 24 to -38 of the maximum. The following table will give some idea of the order of the magnitudes involved ; =r denotes the permeability for p = 0, & the maximum permeability, and |f the force for which it occurs. In some cases the actual maximum is given, in other cases simply the greatest recorded in the tables of experimental results, and the values of |) are stated roughly ; strict accuracy is of no consequence, owing to the great variability of all the magnitudes. Shape. Material. Temper. State.
or W King. Very fibrous iron wire. A i Dialed. Buint. 120 1106 Bar. Soft wire. t1 ,, ISO 24R7 King. Burden s best iron. v Normal. 352 2475 2-5 M ! Magnetic. 216 2459 2 4
V Carefully ami .-: led. Burnt. 544 3621 1-6 t| Norway iron. 11 Normal. 720 5515
M >i Magnetic. 440 4656 12 Bessemer steel. Natural. Normal. 200 1281 5 G Bar. Stubbs steel. 76 331 25-0 Ring. Cast nickel. Annealed. (J 38 169 11-6 at 15 C. Natural. 222 9-1 at 12 C. Magnetic. 224 88 at 220 C. H 314 5-3 Cast cobalt at 5 C. Normal. 142 IS !>
at -5 C. Magnetic. 144 illi 8 " at 2:10 C. " 236 10 1 The smallest permeabilities (for large forces) observe! were for iron 258, for |) = 64 ; for steel 246, for p = 48 for nickel 41, for f = 131 ; for cobalt 55, for |) = 147. 3. The curve showing the relation between K and |(, o between -a and ^, is of such a form that a diameter ca be drawn bisecting chords parallel to the axis of- J[ or g and its equation is approximately ^ , where y K or r, a? = $ or |$, and b,d, B,D are constants. 4. If a metal is permanently magnetized, its permeability is less for low magnetizing forces, but is unaltered for high magnetizing forces. This applies to the permanent state finally attained after several reversals of the magnetizing force ; but if we strongly magnetize a bar in one direction, and apply a weak magnetizing force in the opposite direc tion, the change of magnetization will be very great. 5. Iron, nickel, and cobalt all probably have a maximum of magnetization, although its existence can never be entirely established by experiment, and must always be a matter of inference. If such a maximum exists, then at ordinary temperatures it will be roughly as follows : For iron when ^3 = 17,500, or when f = 1390 ; For nickel when -8= 6,340, or when | = 494 ; For cobalt when = 10,000, or when J = 800. s 2 Baur, Wied. Ann., ii. p. 395, 1880, has remarked that the in tensity of magnetization corresponding to the maximum permeability seems to be about the same for different sorts of soft iron ; e.g., for two of the ellipsoids of Von Quintus Icilius it is 550 and 540 ; for Stoletow s ring, 550 ; for Baur s ring, 540. It would seem that it is much higher for steel, judging by Rowland s tables. 3 The maximum of magnetization for soft iron was calculated from the observations of various experimenters by Von Waltenhofen ( Wien. Ber. t 1869; or Pogg. Ann., cxxxvi.). He finds 1670, or thereby, for the maximum intensity of magnetization. Stefan, using Rowland s graphical method (Wien. Ber., 1874), had found 1400. Fromme (Wied. Ann., xiii., 1881), who had himself actually observed an intensity of as much as 1531, examined the curve for K and |, and found, in agreement withHaubner (Wied. Beibl., v., 1881), that there is a point of inflexion about f = 1200 ; taking this into account, he finds for the maximum value of f 1730, as a mean of results varying between 1720 and 1750. From a result of Weber s (Elec. Maasbest., p. 573) he calculates the value 1737. For the maximum permanent magnetization of steel, Weber (Res. d. Mag. Ver., 1840) gives 314 (common steel magnet); Von Waltenhofen (Pogg. Ann., 1871) 369 (glass hard wolfram steel); Schne beli ( Wied. Galv., Bd. ii. 308) 557 to 671 (sewing needles 25 to 66 mm. long and 6 mm. thick), and 765 to 832 (knitting needles 198 to 210 mm. long and 83 to 175 mm. thick), It must Maxi mum inten sity of field ani of mag netiza
tion.
