on the susceptibility of feebly magnetic bodies as affected by tension

On the Susceptibility of Feebly Magnetic Bodies as Affected by TensionAuthor(s): Ernest WilsonSource: Proceedings of the Royal Society of London. Series A, Containing Papers of aMathematical and Physical Character, Vol. 103, No. 720 (Apr. 3, 1923), pp. 185-189Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/94105 .

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185

On the Susceptibility of Feebly Miagnetic Bodies as affectdcl by Tension.

By ERNEST WILSON, M.Inst.C.E., M.Inst.E.E., Siemens Professor of Electrical Engineering in the Uliiversity of London.

,(Communicated by Prof. 0. W. Richardson, F.R.S. Received November 28, 1922.)

Introduction.

A previous communication* recorded an investigation of the effects of compressive stress upon the susceptibility, retentivity ancl other properties of certain feebly magnetic substances. The present communication is confined to tensile stress, and its effects upon the susceptibility of certain rock specimens, including some of those previously tested under compression. Great difficulty was experienced in the finding of such perfect specimens as are necessary for successful tests of this kind, and even in the best cases the naximum specific loading has been limited to values ranging from about 50 to 130 kgrm. per square centimetre. Throughout the work the susceptibility has been measured in the direction of the stress.

Instrumentation. The apparatus employed in the applicationi of tenisile stress consisted of a

horizontal lever, which was pivoted at one end and weighted at the other. The effective length of the lever was 90 cm. At a point 30 cm. distant from the fuleruin the pull was applied to the specimuen, which was sup- ported by a massive iron tripod. The specimeni had a length preferably of about 6 cm., and its cross-section was square (1 cml. x 1 cm.). The ends were V-shaped, anid accurately fitted into similar gaps in two stout phosphor- bronze rings. The upper ring was attached to a shackle provided with right- and left-hand threaded bolts, the upper bolt being fixed to the tripod, .and the lower ring was fixed to the horizontal lever. By these means the loading could be varied and the shackle allowed of a vertical adjustment.

In order to magnetise the specimen, use was made of a circular stalloy yoke (loc. cit.), whose plane was vertical and at right angles to the lever. The specimen bridged across a horizontal gap 2 cm. wide, which was cut in -the yoke, and, in order to accommodate the test-piece in a central position as regards the opposincg pole faces, a vertical groove 1 cmu. wide was provided. A coil of insulated copper was wound on the yoke, and was connected to a

* 4 Roy. Soc. Proc.,' A, vol. 101, p. 445 (1922).



186 Prof. E. Wilson. On the Susceptibility of

reversing switch in circuit with an adjustable rheostat aild battery of storage cells. An ampere-meter was ineluded in the circuit, and pre- liminary experiments were nmade with a brass test-piece wound with a secondary coil attached to a ballistic galvanonieter, in order to establish the relatioil between the magnetic force H in the gap and the current in the magnetising coil on the yoke. In the actual experiments, the seconldary coil consisted of 600 turns of No. 47 S.W.G. copper wire insulated with silk. The permeability, ,z, of the test-piece was determined from the ballistic galvanometer deflections on reversal of the rlagnetising current, and the susceptibility, Ku, was calculated when required from the formula

-l= I+4rKvK, Longitudinal Susceptibility.

The rock specimens chosen for these experiments consist of magnetite of widely different clharacteristics. They include somiie of those already tested under compression (70e. cit.), and new varieties have been added. The Table gives some detailed information obtained from the experiments.

|Force at Maxi- Kgr/ Coercive B~ orc a Localit;y. [Density. iHmax Coercive Brnax. which K, mum cm./ at- force. Iretained, isamx mu cm2t s a, iSnax. Kv. fracture.

New York State, 4'86 7.0 092 74 10 315 1'46 55 locality unknown 1 300 15 '0 2760 504

569 16 '3 3270 545 New York State, I 4 10 150 - 1960 - 36 1 45 19

(Tilley Foster MVine)F

New York State. 4 58 300 39 2650 830 120 0 '84 29, (Tilley Foster Mine)

Russia, locality 4 -72 300 69 3140 632 150 0 84 50( unknown i

Arkansas ............ 4 '70 300 108 2460 1050 300 0 '56 158 510 141 3500 1410

Manchutria ........ 3-40 525 31 1132 93 140 0,127 123*

Not fractured.

Newv YorZks State (locality unknown). Density, 4'89.-This is a hard andct compact magnetite with no indicatioin of: cleavage planes. It has a bright, irregular fracture, with many glistening points and semi-mnetallic lustre. The

previous experiments showed that the susceptibility of this nmagnetite varied through wide limits as a consequence of compressive stress, and it was to be anticipated that it would exhibit considerable variations under tension. Thi,s. has been found to be the case. Unfortunately, it has not been possible to test it beyond loadings of about 50 kgrm./cm.2, but within this range it has provided sonme interesting results. In the case of iron, when subjected to



Feebly Magnetic Bodies as affected by Tension. 187

varying degrees of tensile stress, it is well kCnown that it is necessary, on account of hysteresis, to distinguish between applying a load which is kept constant, and then taking the magnetic observations, and varyino the load dnring constant ruagnetisation. The actual magnitude of the initial increase in the mag:netic inductioni, due to a change of load, the magnetic force being maintained constant, though present in the case of this magnetite, was not capable of the same investigation which the magnetometer method would have made possible. The galvanometer. deflection, due to the first reversal after a clhange, in load, was greater than the second, and speedily attained, on further reversal, a constant value. Further, the. specific loading, in the case of the muagnetic metals, is enormously greater than is possible with the rock specimens tested, and it has only been possible to investigate the problem over a limited range. Within this range the two methods of procedure, as regards the application of magnetic force a,nd loadinig, gave fairly closely the same results, but there was some evidence of the effect of hysteresis due to load being the same as in the case of iron.

The full line curves in the figure show the percentage change in permeability under a given magnetic force H as the specific load was

H~~~~~~~~~~ wS- - H- 7-7-O- ---4

3-4 ~ ~ ~ ~ ~ ~ ~~~4 .5 | X .0- -----4--- -t

NewYork :Arkansas 243

.;=3 ~ ~~~~~~~~~ ~ ~ ~ ~~~~~~~~~~~~~~~ 7 / | =-1 1 7 ~~~~~~~~325

0t G__-____--X--w S33-

________ -~~~Cm?830

0 20 40 400 60 80 100 120

varied. The increase for a given load rises to a nmaximum when the magnetic force has the value 3-4, and then diminishes. A reversal point is reached when the magnetic force has the value 408, and when the kgrin./cm.2 are equal to 45.

It is of interest to compare the results obtained' when the magnetic force H. = 7 (say) with what happens. in hardened iron.* As the load

* Ewinig's 'Magnetic Induction inl Iron and other Metals,' p. 203.



188 Prof. E. Wilson. On the Susceptibility of

varies in the iron fromn 0 to 880 kgrM./cM.2 the intensity of rnagnetisa- tion varies fronm 280 to 640, and it is substantially proportional to the load. This corresponids to a variation in the magnetic inductioni of 128 per cent. In the present experimnents the percentage increase in the permeability is 6-4 when the load is equal to 50 kgrm./em.2, and for this samyie load the pereentage variation in the iron would be 7 3. The actual permeabilities with no load are 490 in the iroin, and 1-7 in the rmagnetite, and yet they are substantially equal in their sensitivity as regards permeability change.

Another interestinig result is the relationi between the magnetic force at which the chanige in permeability is a maximum, and the force at which the susceptibility is a maximum. The values of H are 3 4 and 315 respectively. The maximum percentage change in the permeability of the iron for a given load also occurs at a lower force than the force at which maximum permeability occurs. The retained magnetic induLction was increased by a load of 52 kgrm./cm.2 to the extent of 8 per cent. when the magnetic force lImax, had the value 7, anid the increase, was 15 2 per cent. when the magnetic force was 400. The coercive force was iniereased about 8 per cent. by the same load wheln the magnetic force Hmax. had the value 400.

New Yor-le State (Tilley Foster Mine).-This variety of mliagnetite is essentially an aggregate of magnetite, calcite and silicates, anid it has an irregular glistening fracture, in which the inclusions can be easily detected. The specimens which have beeni tested show a wide variation in susceptibility, depending to some extent upon the amiiount of the inclusions present. One piece (density 4-10) which was fairly free fromi impurities, but which resembled the magnetite from Hey Tor (loc. cit.), had a high normal maximum susceptibility (1-45), which occurred under the application of a magnetic force H _ 36. It fractured at too low a tension to be of use. Another piece (density 4-58), which was stronger and contained a larger proportion of inclusionis, lhacl a much lower normal maximum susceptibility (0 84), wlhich occurred at a force H = 120. Under a load of 24 4 kgrm./cmY. the increase in permneability reached a maxilum of 8 6 per cent. when the magnetic force had the value 36, and it gradually dropped to 0 6 per cent. at H 300. The specimen was fractured at the next increase in load.

KBtssian (locality unknown). Density 4 72.-This variety is compact with dull irregular fracture and many minute cleavaoes, all in the samlie direction, which in the specimen tested was transverse to the length of the bar. When tested unider a tractive force equal to 45 karmu./cM.2, the maximum increase in the permeability amounted to 5-6 per cent. and the magnetic force had the value 36, which again is well withini the value of the nmagnetic force, H=150, at which maximum susceptibility occurs. The percentage increase in perme-



Feebly Magnetic Bodies as affected by Tension. 189

ability diminished to 0 7 when the force H had the value 150, and it had a negative value of 1-48 under a force of 300 C.G.S. units.

Arkansas. Density 4-7.-This magnetite is hard and compact with irregular cleavage planes, and its fracture is almost vitreous in somne directions anld exhibits a silky lustre in others. The rulagnetite from this district is remarkably uniform, anid differenit specimeiis can be relied upon to give substantially the same results when tested for susceptibility. Its high retentivity is one of its special features. A specimenl of this milneral has been successfully tested at forces up to 127 kgrrm./cm.2, and it fractured at 158 kg,rm./cm.2. This is the highest tractive force at fracture of any of the different varieties of magnetite tested. As might have been anticipated from the compressive tests in which the gradient of susceptibility in terms of load, though small, was well defined, a greater specific load has been required in order to effect a percentage change in permeability comparable with the foregoing varieties. The dotted curves shown in the figure refer to this variety, and should be compared with the full-line curves in this connection.. The percentage increase in the permeability due to traction reaches a, maximum when the magnetic 'force H has the value 243, which is less than the force H = 300, at which the susceptibility reaches a maximum. When. the force H = 830, a reversal point is reached at about 112 kgrm./cm.2.

South73 Manchuria. Density 34.-This is essentially a schist, having a, slaty cleavage, in which the laminated structure is clearly defined. The specimen was cut so as to have the planes of cleavage, in the direction of' pull, and tests were made at various loadings -up to 123 kgrrm./cm.2, the specimen being then unfractured. The results obtained under compression would lead onie to expect little or no variation in susceptibility, and no observable variation was recorded under traction..

Conclusion.

The experiments demonstrate that when magnetite is subjected to tensile stress ranging from about 50 to 130 kgrm. per square celntimetre as a. maximum., the susceptibility for a given value of the magnietic force at first increases, and then decreases as the specific load continuously iniereases, and exhibits a reversal point as in iron. The magnetic force at which the percentage increase in permneability has a maximum value is less than the magnetic force at wlhich maximum susceptibility occurs. The specimens tested have widely different structural characteristics.

The author would like to express his thanks to Prof. W. T. Gordon and Prof. E. F. Herroun for their kind help during the research.



on the susceptibility of feebly magnetic bodies as affected by tension

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