Permanent Magnets. 143 



x is the length of the separate bar, 

 I „ „ „ whole bar, 

 M, total moment of m bars of n pieces each, 



— , moment of one bar of n pieces, 

 m' 



§3, magnetic induction (lines of force per unit area, field- 

 intensity) through equatorial section of bar of n pieces, 

 K radius of bar, S = 7rR 2 , 



_, 4-7T moment P T ,. , * 



F= o** = local distance*, 



fow 



/ = — = focal distance, length unity. 

 x 



Bar A. 



N=18. I =28-65 centim. R=«978 centim. 



n . ... 1. 2. 3. 6. 9. 18. 



TvrfObs. ... 418 687 928 1773 2372 3494 



M \Calc 418 686 954 1758 2562 4974 



Obs.-Calc. ..0 +1 -26 +15 -190 -1480 



M 23-2 76-0 154-7 591-0 1186 3494 



m 



_ r Obs 38-9 74^ 123-1 275-4 361 594 



*3 ^ Oorr 46 92 138 276 



f T33 T09 ^982 -938 -959 -859 



F 211 3-45 4-69 8-92 13-7 24-6 



M calc. = 150 + » 2681 to n==6 . 

 $3 corr. = n 4b J 



The limiting value of F when w = is, according to these 

 expressions, F =*758 centim. 



^ — •775 



(As to the calculation of F see post. (2).) 



The total moment has a finite but small limit ( = 150), when 

 the bar is supposed to be divided into an infinite number of 

 thin disks. As to the persistence of the linear laws in the case 

 of actual disks of small thickness, see bars B and C post. 

 The moment increases with increments proportional to those 



* The points thus defined would he the poles, if the conception of poles 

 were true and applicable ; in bars the distance of the foci is usually a 

 little less than that of the poles as ordinarily defined. In the present 

 paper the foci are made use of as indicating by their position the distri- 

 bution of the magnetism. They may be also regarded, with reserve, as 

 the points of application of the resultants of the lines of force. 



