106 ELEMENTARY LESSONS ON [CHAP. ,11. 



amounts of the forces may be represented by certain 

 lengths marked out along these lines. Suppose the 

 distance P N is twice as great as P S, the repelling force 

 along P N will be ^ as strong as the attracting force 

 along PS. So measure a distance out, P A towards S 

 four times as long as the length P B measured along P N 

 away from N. Find the resultant force 1 in the usual 

 way of compounding mechanical forces, by completing 

 the parallelogram PARE, and the diagonal P R represents 

 by its length and direction the magnitude and the 



Fig. 59- 



direction of the resultant magnetic force at the point P. 

 In fact the line P R represents the line along which a 

 small magnet or an iron filing would set itself. In a 

 similar way we might ascertain the direction of the lines 

 of force at any point of the field. The little arrows in 

 Fig. 59 show how the lines of force start out from the N. 

 pole and curve round to meet in the S. pole. The 

 student should compare this figure with the lines of 

 filings of Fig. 50. 



1 See Balfour Stewart's Lessons in Elementary Physics, page 26 ; or 

 Todhunter's Natural Philosophy for Beginners, page 55. 



