MAGNETISM 269 olume. original length. This residual extension was due in part to permanent magnetism, but he found that the permanent magnetization due to a current 1088 was reversed from - T3 to + 25 by a current 175, while two-thirds of the permanent extension was still left. The actual elongation of an iron bar magnetized to saturation was found to be from T . 20 1 Q( . )0 th to Ty-^jj-j^jth of its whole length. The exten sion varied approximately as the square of the intensity of magnetization (temporary or permanent). The general character of the phenomena is the same in soft or hard iron, and in soft or hard steel ; l but the effects are smaller with hard than with soft bars. ffect of It was found that longitudinal compression of the bar
- nsion. influenced the magnetic extension little if at all ; on the
other hand, longitudinal traction was found to diminish it, and in the case of thin wires under considerable tension the magnetization caused a contraction. Thus in the case of a bar 1 foot long, ^ inch in diameter, with a weight of COO ft), there was neither extension nor contraction, even with a current of 1600; with weights of 1040 tt) and 1080 ft), and a current of 1804, there was a contraction of 00002 inch and 000032 inch respectively. The contrac tion under tension was found to vary approximately as the product of the magnetizing current and the intensity of magnetization. After the magnetizing force was withdrawn the wire regained its original length, permanent magnetiza tion notwithstanding.
T o alter- Joule made careful experiments to determine whether
tion of t] ie magnetization of an iron bar produced any alteration of its volume, but could find none. He therefore concluded that the longitudinal extension of a magnetized bar is accom panied by an equal lateral contraction ; and, in accordance with this conclusion, he found that when an iron tube is cir cularly magnetized, perpendicular to its length, by passing a current along its axis, it contracts longitudinally. The results of Joule have been verified and to some extent added to by Wertheim, 2 Buff, 3 Beez, 4 Tyndall, 5 Mayer, 6 Righi, 7 and Ader. 8 The three first experimented with magnetizing coils shorter than the bar, and found that the extension was much greater when the coil was near the free end than when it was near the fixed end of the bar. This of course raises the question how far the extension is due to electromagnetic action between the coil and the bar, and how far to internal molecular disturbance. 9 Mayer s results are in agreement with Joule s except in the case of bars of soft steel, which (not under traction) when the magnetizing current was first estab lished, elongated in some cases and retracted in others, at the first break elongated, and subsequently retracted at make and elongated at break. Righi s results for longitudinal magnetization are in agreement with those of Joule ; he also gives a variety of interest ing results regarding the effects of circular and longitudinal mag netization on the length of iron wires. Barrett 10 has recently arrived at the interesting result that nickel behaves oppositely to iron, retracting about rmnnnyth when magnetized to saturation ; he gives for the elongation of iron and cobalt under like circum stances "nnHnnr th and TWrnnrth respectivel} . Effect of Effect upon Magnetization of Traction along the Lines of traction Magnetization. Matteucci 11 seems to have been the first to discover that when a bar subject to a magnetizing force in the direction of its length is stretched in the same direction its temporary magnetization increases. When the stretching force is removed the magnetization again diminishes. Wertheim 12 confirmed Matteucci s observation. Villari, 13 however, found that, after the first effect, which on mag netiza tion. Mat teucci. Villari. 1 In the case of a bar of hard steel he found a considerable increase in length every time the magnetizing current was interrupted. This he attributes to a state of "tension " in the hardened steel. 2 Ann. d. Chim. et d. Phys., 1848. 3 Cited by Wiedemann, Oalv., ii. 504. 4 Pogg. Ann., 1866. 5 Diamagnetism and Magnecrystallic Action, 1870. 6 Phil. Mag., 1873. 7 Nuov. dm., 1880. 8 Comptes Rendus, 1880. 9 See Wiedemann s remarks, Galv., ii. 503. 10 Nature, vol. xxvi., 1882. 11 Comptes Rendus, 1847 ; Ann. d. Chim. et d. Phys., 1858. 12 Ann. d. Chim. et d. Phys., 1857, 1858. 13 Pogg. Ann., 1868, also 1865, 1869. is always increase, the application of the traction will cause increase if the intensity of magnetization is not beyond a certain critical value, but decrease if that value is surpassed ; the removal of the traction causes in each case the opposite effect to the application. The effect of the first traction on the permanent mag netization, whether of iron or steel, is a diminution ; the effect of subsequent tractions in steel is a diminution on application, with increase on removal ; in soft iron an increase on application, a diminution on removal. Partial demagnetization of a steel bar by an opposite magnetic force causes it to behave like soft iron ; when the demagnet izing force is sufficient to reverse its polarity, the effect of even the first traction maybe to increase the magnetization. Sir W. Thomson 14 has carefully studied the phenomena Thom- in question, as exhibited in a very soft iron wire 075 cm. son. in diameter permanently stretched by a weight of 1 tt>, and alternately stretched by weights of 7 ft), 14 ft>, or 21 ft), and unstretched (so that there was no permanent elongation). As the magnetizing force was increased, the increase of magnetization caused by the application of traction increased to a maximum, then diminished, and became zero for a certain critical value of the magnetizing force ; after the critical value was passed, the traction caused a diminution of the magnetization, which increased asymptotically towards a fixed limit as the magnetizing force was increased, more and more. The following table will give an idea of the results. I denotes the maximum increase, and D the limit of the decrease, roughly estimated in the same arbitrary unit ; |j is the force corre sponding to I, and P the critical force, each expressed in terms of the earth s vertical force at Glasgow as unit ; T is the traction, t the temperature. T t I D
io 7 Orel. 100 +31 +26 -6 -3 5-9 6-4 34 35 1 Ord. 100 +35 +32 -14 -9 4-8 4-5 25 25 21 Ord. 100" +54 +51 -21 -15 4-5 5-0 26 29 Bars of nickel and cobalt were also examined ; and it was found that after the first effect the result of applying traction in the direction of magnetization was in both cases to diminish the magnetization. The effect appeared to increase up to a maximum, and then to diminish as the magnetizing force increased ; but the critical value was not reached with the largest forces employed. Traction perpendicular to the lines of magnetization was found by Thomson to diminish the magnetic susceptibility. The experiment was made by means of a gun barrel magnetized longitudinally, and subjected to internal bydro- static pressure. .The effect of pressure along or perpendicular to the magnetization would in all probability be opposite (and equal 1) to that of an equal amount of traction ; but no experiments have as yet been made on the subject. 15 The effect of traction is therefore to produce magnetic seolotropy, the susceptibility being increased in the direction of the stress and diminished in the perpendicular direction so long as the intensity of magnetization is not above a certain critical value ; above that value the effects are reversed. The effect of pressure would be opposite in every particular. Hence the effect of a shearing stress would be increase .of magnetic susceptibility along the principal axis of elonga tion, and decrease (to an equal extent?) along the principal axis of compression. 14 Phil. Trans., 1876 and 1879, p. 55. 15 Pressure applied outside Thomson s gun barrel would enable us to observe the effect of transverse pressure ; and by magnetizing the barrel circularly the effect of pressure along the lines of force could be
determined.