Self-Demagnetizing Factor of Bar Magnets. 73& 



by experiment) with the 33-2io curve of a ring (or infinitely 

 long rod) of the same iron, and taking the difference of the 

 values of <?& for some assigned value of <%>. On the other 

 hand, values of c/ can be found by experiment, either 

 magnetometrically, giving the mean value, or ballistically, 

 caving either maximum or mean according as whether the 

 exploring coil on the bar is wound over its whole length or 

 over its equatoreal zone only. The ratio ^o d -^ J so deduced 

 may still be called the self-demagnetizing factor, and values 

 found for rods of different dimension-ratios. 



Magnets of other forms, for example the slit toroid, or 

 anchor-ring with a gap in it, and the horse-shoe magnet with 

 parallel limbs of given proportions, will likewise have self- 

 demagnetizing factors of their own, dependent on their 

 geometry and on the distribution of their polarities. With 

 them also, neither J^ d nor c/ will have constant values at all 

 points within the substance of the magnet ; and for each form 

 therefore the term " self-demagnetizing factor " bears a 

 significance different from that which it possesses for the 

 ellipsoid of revolution or for the cylindrical bar. 



All previous writers have defined the term dimension-ratio 

 as applied to a bar as the ratio between its length I and the 

 diameter d of its circular section. But when we come to 

 deal with forms of cross-section other than circular, it is 

 inconvenient to use this mode of expression. For if we were 

 dealing with a flat bar of breadth 6, the curve for self- 

 demagnetizing factors in terms of the ratio l-=rb would not be 

 comparable with those for cylindrical bars in terms of l-^-d. 

 The preferable way, when such comparison has to be made, 

 is to state a dimension-ratio, for bars of all and every form 

 of section, in terms of the ratio which is borne by the length 

 to the square-root of the area of section. The ratio Z-h^/A, 

 wo accordingly propose to denote by the symbol \. For any 

 given bar we have the relation X = ttt x 1*128. 



Part II. — Experimental. On the Values of the Self- 



DEMACxNETIZING FACTOR FOR BAR-MAGNETS OF CIRCULAR 

 SECTION. 



Several investigators, including Ewing, Fromme, Holz, 

 and Ascoli, have written on the factor of self-demagnetization 

 of cylindrical bar-magnets, and have given experimental 

 values for bars having different ratios of length to diameter. 

 The best-known results are those published by Du Bois, who 

 has compared the values obtained with those for ellipsoids 



