we get 







Fig. 1 (PI. XV.) gives the «^— ^curves for our rods, the 

 dimension-ratios of which varied from 35'6 to 2' 66. These 

 curves were sensibly straight lines up to <%-=. 12,000, or as high 

 asjthe curves could be carried. The value ^=10,000 was 



Self -Demagnetizing Factor of Bar Magnets. 735 



magnetization of the bars. On the middle of each bar was 

 wound an exploring coil of 10 turns of very fine wire, the 

 breadth of each such coil not exceeding 0*25 cm. The 

 galvanometer was calibrated by the short coil previously 

 mentioned, its dimensions being accurately known. The 

 magnetizing current was measured by a standard commercial 

 amperemeter, the readings of which were calibrated at regular 

 intervals of time by a Crompton potentiometer. 



Each specimen was mounted on a carrier by means of 

 which it could be inserted centrally in the middle of the long 

 magnetizing coil. The galvanometer calibration having been 

 effected, a test was made of each bar by subjecting it to a 

 series of reversals in fields varying from #zf=20 to ^=255, 

 the throws of the galvanometer being noted; and for each 

 bar a 3B-2& curve was then plotted. 



A similar curve having been plotted from the tests made 

 on the ring, the values of the demagnetizing intensity of 

 field %C di due to the self-demagnetizing action of the poles 

 of each bar, could then be calculated, for any value of ffi, by 

 taking the abscissa, corresponding to that ordinate, in the 

 curve for that bar, and subtracting the corresponding- 

 abscissa in the curve for the ring. 



Let the field due to self-demagnetization at the mid-point 

 of any bar, for any given flux-density «^, be called i%. Let 

 the total impressed field due to the magnetizing coil be called 

 %C\ and let the impressed field required in the ring to pro- 

 duce the same given value of $ be called $f r . Then 



Then since, by definition, the self-demagnetizing factor N 

 has the value 



and 



