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THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XX.— On the Permeabilities and the Reluctivities, for 

 very Wide Ranges of Excitation, of Normal Specimens of 

 Compressed Steel, Bessemer Steel and JVorivay Iron Rods ; 

 by B. Osgood Peikoe. 



When a rod or a closed frame of iron becomes magnetized 

 under the action of a steady electric current in an exciting 

 coil of insulated wire wound about it, the flux of magnetic 

 induction (B) through any cross section of the iron can be 

 easily determined with the' aid of a small testing coil, but it is 

 often very difficult to tell just what the value of the exciting 

 magnetic' field (B) is at any given point within the metal. In 

 a few familiar cases, however, the difficulty disappears. 



If, for instance, a homogeneous round rod of soft iron the 

 length of which is,. say, hVe hundred times the diameter, be 

 placed in a solenoid of narrow bore, somewhat longer than the 

 rod and uniformly wound with n turns of insulated wire per 

 centimeter of its length, and if a steady current (G) be sent 

 through the solenoid, the demagnetizing effects of the ends of 

 the rod become inappreciable near the center, O, and we 

 may assume without sensible error that the value of B at every 

 point of a cross section in the neighborhood of O is equal to 

 the value (^irnC/10) of // just without the rod at O. 



In the case of a soft iron toroid, uniformly wound with JV 

 turns (in all) of insulated wire, the value of Bis, of course, 

 not the same at every point of a meridional section of the 

 metal, but if the material is perfectly homogeneous, there is 

 practically no leakage of induction into the air. Under 

 these circumstances, the magnetomotive force is the same 

 (■i-n-jVC/lO) around all closed non-evanescible paths in the 

 iron, and the value of B is inversely proportional to the dis- 

 tance from the axis of revolution of the toroid. In practice, 

 kit Jour. Sci.— Fourth Series, Vol. XXVII, No. 160.— April, 1909. 

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