. A text book of physics, for the use of students of science and engineering . f affairs is represented diagrammatically in Fig. 753, wherethe original field H is represented in dotted lines, and the inductiondue to the magnetisation of the iron by continuous lines. In order to find the total value of the induction due to the magneti-sation of the iron, consider a section of it in the inter-molecularspaces at P. If I be the intensity of magnetisation of the iron, then on each side of the sectionthere is an amount of pole I unitsper square centimetre, N on oneside and S on the other, andthe num
. A text book of physics, for the use of students of science and engineering . f affairs is represented diagrammatically in Fig. 753, wherethe original field H is represented in dotted lines, and the inductiondue to the magnetisation of the iron by continuous lines. In order to find the total value of the induction due to the magneti-sation of the iron, consider a section of it in the inter-molecularspaces at P. If I be the intensity of magnetisation of the iron, then on each side of the sectionthere is an amount of pole I unitsper square centimetre, N on oneside and S on the other, andthe number of lines crossing thissquare centimetre is 4ttI (p. 811).Hence, the total number of linesof induction per square centi-metre (B) is made up of H dueto the original field, and 47rldue to the magnetisation of the Fig. 753.—Lines of induction due to a iron or B=H + 4~ material. In the case of a permanentmagnet there may not be any magnetising field, in which caseB = 4ttI. The above expression may be written differently ; for, on dividingthrough by H, we get b I. H = 1+^H: or jbt = 1 + 47T&, li being the magnetic permeability of the material, and k itsmagnetic susceptibility, as defined on p. 811. The right-hand side of the equation B=H+4ttI is representedby the lines drawn in Fig. 753, the two sets of lines being drawn inthe same diagram. Within the iron we see that their resultant,or sum represents B. At external points the two sets are not in thesame direction at every point, and in order to obtain their resultant,the two fields must be compounded by the ordinary law for theaddition of vector quantities. This is not always easy, particularlyfor a rectangular bar, but the general arrangement is exhibited in|,V-- 754. It will be noticed on examining this diagram of theresultanl field, that the effect of placing the iron bar in a uniformfield is to cause the lines of induction to converge upon it, producing LXIV SOFT IRON KEEPERS 817
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