M-AGNETIS M 255 A set of observations on ellipsoids of revolution were made by Riecke, 1 by the method just described. The ellipsoids, seven in number, were all cut from tl*e same piece of soft iron, but varied in volume and in eccen tricity. The resulting values of K were found to be independent of the volume of the ellipsoids and of the part of the iron from which they were cut ; but, on the other hand, with one slight exception, they increased with the eccentricity of the ellipsoids. Kohlrausch, in com municating these results to Poggendorjf s A nnalen, remarked that they stand in contradiction with the theory of Poisson and Neumann ; in so saying he probably considered the constant vertical force of the earth (p ) to be the argument of the function K but this is not so, as Stoletow points out in the paper already quoted. The actual magnetizing forces are greater in the more elongated ellipsoids ; and Riecke s results simply prove that for values of H varying from 031 to 072 K increases from 13 5 to 254. In order to establish the initial increase of the magnetization function K beyond all doubt, Stoletow (/. c.) made a new set of experiments on a carefully annealed iron ring 2 of rectangular section (exterior diameter 20 cm., in- terior diameter 18 cm., height 1-47 cm.). The ring was care fully wound throughout with a primary coil of n ( = 800) windings; over this, in one or more shorter or longer stretches, was wound a secondary coil of n ( = 5Q to 750) windings. The induction current in the secondary, due to the reversal of a known current i in the primary, was sent through a galvanometer, and thus measured. If E be the electromotive force of this current, then (see above, p. 246) E = 4n t(47ricM + P), where M and P can be cal culated from the dimensions of the ring and its primary coil. All then that is necessary is to know E/i in absolute measure. We refer the reader to the original paper for the details of the measurements. The results are very interesting, and fully confirm the conclusions drawn from the results of Von Quintus Icilius and Riecke. The smallest value of |j was 43, and the corresponding value of K 21 5 ; the maximum value of K was 174, for ^ = 3 2; the last value observed was K = 42 l, for |) = 30 7. The temperature varied from 15 C. to 20 C., but it appeared from the experiments that K did not alter much for moderate changes of temperature. In figure (34) is given a transcription of the curve that represents the results of Stoletow s experiments ; the abscissae represent the values of P in C.G.S. units, and the ordi nates the correspond ing values of K. About the same time as Stoletow, and independently, Rowland 3 made a much more extensive series of experi ments, the results of which form one of the most important contributions yet made to our knowledge of magnetic induction. The experiments were made partly on very long bars; but the published results were mostly obtained from rings, it having been found that the effect of the ends of the bar was sensible even when the length was as much as 144 times the diameter. About a dozen rings of iron, nickel, and cobalt were used ; the section was circular in all cases ; and a primary and a secondary coil were used as in Stoletow s experiments. The primary current was measured by means of a tangent galvanometer in which 1, 3, 9, 27, or 48 coils could be brought into operation according to the sensibility required. The induction current in the secondary was measured by the swing of a Thomson s galvanometer fitted with a heavier needle than usual. The indications of this last were reduced to absolute measure by taking the swing caused by turning over a 1 Pogg. Ann., cxli., 1870. 2 A method suggested by Kirchhoff, Pogg. Ann., Ergbd. v., 1870. 8 Phil. Mag., 1873, 1874. 34. horizontal coil of known area, inserted in its circuit, so as to produce the full induction current due to the earth s vertical force. In order to obtain the total induced magnetization the primary current was reversed. To obtain the permanent magnetism it was simply broken ; this gives the part of the induced magnetism that disappears with the inducing force (temporary magnetism Rowland calls it) ; subtracting this from the total mag netization, we get the permanent magnetization. Care was taken in these experiments always to work with magnetizing forces of ascending magnitude, as it was found that the effect of any force is considerably modified if a greater force has previously acted on the body, in other words, that the magnetic permeability of iron or steel is much affected by pre-existing permanent magnetism. This fact raises an objection to the ring method ; for per manent magnetization in a ring is not easily discoverable, and would give it a one-sidedness, so that a magnetizing force would produce much more alteration when exerted in one direction than it would when exerted in the other. 4 Rowland publishes about thirteen different tables, relating to rings of iron and steel in different states, and also to nickel and cobalt, under different conditions as to temper.
4 See Rowland, Phil. Mag. (4), 48, p. 336 ; also above, r- 254.