CHAP. IT.] ELECTRICITY AND MAGNETISM. 91 



in the field, the resultant direction of the forces at every 

 point ; for each particle tends to assume the direction of 

 the magnetic induction due to the simultaneous action of 

 both poles ; hence they may be taken to represent the 

 lines of magnetic indiiction.' 1 Faraday pointed out 

 that these " lines of force " map out the magnetic field, 

 showing by their position the direction of the magnetic 

 force, and by their number its intensity. If a small N.- 

 seeking pole could be obtained alone, and put down on 

 any one of these lines of force, it would tend to move 

 along that line from N. to S. ; a single S. -seeking pole 

 would tend to move along the line in an opposite direc- 

 tion. Faraday also assigned to these lines of force 

 certain physical properties (which are, however, only 

 true of them in a secondary sense), viz., that they tend 

 to shorten themselves from end to end, and that they 

 repel one another as they lie side by side. The modern 

 view, which holds that magnetism results from certain 

 properties of the "ether" of space, is content to say 

 that in every magnetic field there are certain stresses, 

 which produce a tension along the lines of force, and a 

 pressure across them. 



1O9. This method may be applied to ascertain the 

 presence of " consequent poles " in a bar of steel, the 

 figure obtained resembling that depicted in Fig. 51. 

 Such a state of things is produced when a strip of very 

 hard steel is purposely irregularly magnetised by touching 

 it with strong magnets at certain points. A strip thus 

 magnetised virtually consists of several magnets put end 

 to end, but in reverse directions, N.-S., S.-N., etc. 



HO. The forces producing attraction between unlike 

 poles, and repulsion between like poles, are beautifully 

 illustrated by the magnetic figures obtained in the fields 

 between the poles in the two cases, as given in Figs. 



1 Or rather the component part of the magnetic induction resolved into 

 the plane of the figure ; which is not quite the same thing, for above the 

 poles the filings stand up nearly vertically to this plane. 



