OF ARTS AND SCIENCES. 271 



XX. 



ON CHANGE OF FORM AFFECTING A MAGNETIC 



FIELD. 



By a. Emerson Dolbear. 



Presented January 14, 1891. 



Hitherto, the study of a magnetic field has been the study of the so- 

 called lines of force radiating from the poles of magnets, either electro 

 or permanent ; and, so far as magnetism has been utilized in the arts, 

 the changes in this external field have been brought about by the 

 movements of an armature, having for its function to determine the 

 direction and consequent density of the field. Such is the case in 

 the instruments used in the telegraph, the telephone, in dynamos, and 

 in motors. Sometimes, conducting wires are so mounted in the field 

 that their movement gives rise to electric currents ; which signifies 

 that the energy producing the tension in the field is absorbed in some 

 measure by the moving wires, and is transformed into an electric 

 current. In each of these cases the magnet producing the field is 

 stationary ; that is, changes in the magnetic field produced by it are 

 due to a motion external to the magnet itself, and may be that of an 

 armature, of a moving wire, or of its own bodily change of position, — 

 a kind which is comparable with what is called external motion in 

 thermodynamics, to distinguish it from internal motions, or such as 

 take place when the body changes its form. So far as I am aware, no 

 study has been made of the eflFect of changing the form of a magnetic 

 body on its field, or of the reaction upon itself of its magnetic condi- 

 tion due to a periodic change of form. Of course, it has been known 

 for a long time that the form of the magnetic field depended upon the 

 form of the magnet itself. For a straight bar magnet, this field is 

 familiarly known by the arrangement of iron filings forming curved 

 lines from each pole re-entering the opposite pole. When the iron 

 is bent into a U-form, or horseshoe magnet, the field is mostly con- 

 tracted to the space between the poles. These forms of magnets 

 have been permanent ones for the purpose for which the magnet was 

 made. 



