OF ARTS AND SCIENCES. 



385 



No. III. — ON THE EFFECT OF ARMATURES ON THE MAGNETIC 

 STATE OF ELECTRO-MAGNETS. 



Bt B. O. Peirce and E. B. Lefavopk. 



M. Jamix has lately shown that the effect in providing a steel magnet 

 with an armature consists merely in a redistribution of magnetism, but 

 not in an increase. The following experiments were instituted, to 

 determine what was the effect of armatures of electro-magnets on their 

 magnetic state. The method of experimenting was to slip a coil of fine 

 wire over the electro-magnet, which was provided with a scale, and to 

 measure by the swing of the needle of a reflecting galvanometer the 

 induction currents which arise on making and breaking the circuit of 

 the electro-magnet. The first experiments were made with a straight 

 electro-magnet, 19 cm. long, 1.5 cm. wide. The core consisted of a 

 bundle of fine iron wires, the. ends of which were filed in one plane, 

 upon which the armature, which consisted of a piece of iron 6 cm. long, 

 1.5 cm. wide, rested. It was found that the mass of this armature made 

 no difference, as long as the end of the core of the electro-magnet was 

 completely covered, and in close contact with the armature. The fol- 

 lowing table gives the results obtained. Only the currents produced 

 by breaking the primary circuit are given. 



TABLE L 



"When these results are represented by curves, they show that the 

 magnetic state, in a straight electro-magnet without an armature, 

 VOL. X. (n. s. II.) 25 



