OF AKTS AND SCIENCES. 207 



lously under the influence of reversals of naagnetism, and it is sup- 

 posed that the particles of the entire mass of the iron partake more or 

 less of this movement. A musical note, however, is given forth by 

 the coil of the electro-magnet before the iron core is inserted, when 

 an alternating current is passed through the coil. This musical note 

 is very weak, and is greatly strengthened by the insertion of the iron 

 core. This note can arise from the attraction of the spires of the coil, 

 which communicate a vibrating motion to the air in the neighborhood 

 of the coil ; and this action is much increased by the inductive action 

 caused by the insertion of the iron core. 



If we accept Weber's theory, and suppose that the heat developed 

 is due to a movement of the molecules, it follows that, if a piece of 

 iron is alternately magnetized in different directions, the movement 

 of the molecules, and the resultant heat, must be proportional to the 

 angle between the directions of magnetization. Our experiments 

 were made to test this conclusion. 



A circular disk of soft iron was placed between the poles of two 

 electro-magnets. A current was passed alternately through the mag- 

 nets. The rise of temperature of the disk, after a certain time, was 

 measured, when the axes of the poles were in the same straight line, 

 and when they were inclined to each other at an angle a little less 

 than 90°. According to the general theory, the heating should be less 

 in the latter case. 



Two disks cut from different pieces of soft iron were used. The 

 diameters were respectively 2.7 cm. and 1.3 cm. ; the thickness, 0.1 cm. 



In order to make the magnetic field of the same strength in the two 

 positions of the magnets, shunts of suitable resistance were applied to 

 the circuits when the axes of the magnets were in line. The inten- 

 sity of the field of each magnet was measured, in the two positions, 

 by rotating a coil of fine wire. The arrangement of the shunts was 

 found to answer perfectly ; the intensities being unaltered by change 

 of position. In the first series of experiments this precaution was 

 neglected, and there was consequently a greater heating effect in one 

 case than in the other. 



The arrangement for reversing the direction of magnetization con- 

 sisted of a large disk of cherry wood, 91.5 cm. in diameter, and 5 cm. 

 in thickness. Around the circumference of the disk were listened, at 

 equal distances apart, 104 strips of sheet brass, 1.35 cm. Avide. The 

 disk was placed upon one end of an axle, the other end of which car- 

 ried an iron wheel about 60 cm. in diameter. The iron wheel could 

 be turned by a handle fastened to one of the spokes. All the brass 



