. A text book of physics, for the use of students of science and engineering . oles is m^jud2 dynes, and the strength of field (H)due to mx is mj/xd2. Thus the strength of field due to a pole dependsupon the medium in which it is situated. It is most important, aswe shall see later, to have some quantity which is fixed for a givenpole and distance, independently of the medium. This quantity iscalled the magnetic induction (B), an,d we see from the above that itmust be /u times the strength of field, or B = ^H. Now, for a distance d from a given pole, H_ m . ? _ >» m/*& B~UjIdz-¥B is therefo
. A text book of physics, for the use of students of science and engineering . oles is m^jud2 dynes, and the strength of field (H)due to mx is mj/xd2. Thus the strength of field due to a pole dependsupon the medium in which it is situated. It is most important, aswe shall see later, to have some quantity which is fixed for a givenpole and distance, independently of the medium. This quantity iscalled the magnetic induction (B), an,d we see from the above that itmust be /u times the strength of field, or B = ^H. Now, for a distance d from a given pole, H_ m . ? _ >» m/*& B~UjIdz-¥B is therefore independent of the medium, thus fulfilling the condi-tion required. Thus, the definition of magnetic induction is that it isif times the strength of magnetic field at any point where H is defined ason p. 781 and ju as defined on p. 811. Magnetic lines of induction.—In plotting lines of force (p. 778)the process always begins at the surface of the magnet, in fact may be said to arise upon N poles and end on s poles. TheseJ LXIV MAGNETIC INDUCTION 813. Fig. 749.—Lines of induction. lines, however, are continuous with others within the magnet, sothat each line in its entire path forms a closed circuit (Fig. 749);but within the magnet they are not lines of force, that is to say,they do not by their number per unit area, represent the strengthof field, or force that would beexerted on a unit pole. Theseare magnetic lines of induction,and may, or may not, lie whollyor partly within a magneticmaterial. When in air or othernon-magnetic material they arealso lines of force ; that is, theyrepresent the strength of fieldby their number per unit area,but do not do so within amagnetic material. The num-ber of these lines per squarecentimetre, however, represents the magnetic induction at the placeconsidered, whatever the nature of the medium may be. Therelation of their number per unit area to the strength of field withina magnetic material will be found on p. 816. Gausss
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