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PROCEEDINGS OF THE AMERICAN ACADEMY. 



ent, the value of B is at its greatest smaller than in the case of the 

 solenoid magnetization. 



Figure 7 shows in the curve PADQ the distribution of magnetism in 

 an unequally hardened cast-iron rod when the magnetization took place 

 in a long solenoid. Curve PDBQ shows on an exaggerated scale the 

 distribution when the rod was magnetized between the poles of a large 

 electromagnet. The greatest value of B was in this latter case about 

 two thirds the corresponding value when the solenoid was used. In 

 all the instances I have met, the solenoid gave the greatest value of B 



Figure 7. 



and any other distribution gave an appreciably smaller value. Table 

 IX gives the resistivity at points distant n cm. from the end of the rod 

 which corresponds to G in Figure 7. It is evident that one end of the 

 rod is glass-hard and the other very soft. 



The most common form of irregularity in a cast-iron bar magnet 

 seems to consist, if one may judge from a filing diagram, in a simple 

 displacement of the magnetic centre from the geometric centre towards 

 one end of the axis. This usually corresponds to a comparatively slight 

 difference of resistivity along the bar. This case may be illustrated 

 by a rod (K) which had once been hardened irregularly and then had 

 been rehardened as uniformly as possible. In all such cases it is ex- 

 tremely difficult to get rid of the effects of careless hardening, though 

 the irregularity may come up in a slightly different form. The next 

 table (X) gives the resistivity of the metal, and, on an arbitrary scale, 

 the value of ^ at a point distant n cm. from one end of this bar, which 

 was 29 cm. long. 



Table XI gives the resistivities and the relative values of B on the 

 axis of a cast-iron magnet (Q) made of a rod hard in the middle and 

 soft at the ends. 



