330 PROCEEDINGS OF THE AMERICAN ACADEMY 



same speed of the commutator. A steel bar, one half inch in diameter, 

 gave nearly the same results as the one fourth inch rod of soft iron, but 

 the highest velocities were not used with any of them. It was, how- 

 ever, determined for both that an increase of electromotive force causes 

 a considerable increase in the retardation, but apparently affects the 

 soft iron more in the nearer, and the steel in the further, portions. 



Little reliance can be placed in determinations for distances above 

 20 inches ; but it would seem probable, from the experiments below, 

 that the anomalous behavior of the steel is due to "coercive force," 

 which in the most distant parts gives way only under the influence 

 of a powerful battery, while that of the soft iron is due to super- 

 saturation, which can exist only in the nearer portions. 



It appears, also, from the experiments, that the " magnetic conduc- 

 tivity " of the steel cannot be far from a fourth of that of the soft iron.* 



TABLE FOR HALF-INCH STEEL ROD. 

 F = 81 - - - 86. 



* See " Rowland's Tables of Permeability of Steel and Iron," American 

 Journal of Science, 1873. 



