152 On Permanent Magnets. 



Then if, as before, a bar of length I be divided into m pieces, 

 so that #= — ? 



P'S I 2 



Moment of one piece. . = -. 5 , 



r 47r/> m" 



P'S Z 2 

 Total moment of w pieces = -. 



47T/3 m 



Although it would probably not be easy to determine F by 

 independent reasoning of a simple character, it is not difficult 

 to see that it must be the sum of F and some fraction of a 

 such as *8 or *9 ; and, admitting this, we obtain the form of 

 the general expression for the moment within the linear 

 range, thus : — 



Moment = — = — 

 4tt 



= 4^( F o + ^)say, 



which fairly represents the actual expressions. 



In the region beyond the linear range ihe magnetic resist- 

 ance increases; the fall of both moments and inductions below 

 the linear values (see figures to bar A) can be represented as 

 due to this, as is clear from the formula last written down. 

 Some values of the magnetic resistances are subjoined. 



Bar A. 



Magnetic Resistance of Bar of n pieces. 



n 1, 2, S, 6. 9. 18. 



Calc r From disk-moment . . -758 '869 1-056 



' \ From demagnetization *909 



Induction ( pieces in right order -891 



under R,< , . 



100coils| reversedm P airs i'Oll 



By induction produced in demagnetization, 10 coils '928 



Formula for resistance from disk-moment . = — ~xz- > 



, ,. ,. 47rx43 

 „ „ „ demagnetization = =^— . 



There is a fair accordance considering the difficulties intro- 

 duced by the want of uniformity. An endeavour will be made 

 to obtain a magnet in more uniform condition. 



