184< Prof. A. M. Mayer on the Magnetic Elongations 



firm the result arrived at by those experimenters. Recently 

 Cazin (Comptes Rendus, vol. lxxv. p. 1265) has shown that " the 

 heat thus produced is proportional to the square of the intensity 

 of the magnetism and to the polar distance;" and Moutier 

 (Comptes Rendus, vol. lxxv. p. 1620)* deduces this result from 

 a thermodynamic theorem established by Clausius, and thus 

 concludes his paper: — "The increase of vis viva which the bar 

 experiences from the effect of magnetization is therefore propor- 

 tional to the square of the intensity of the magnetism and to the 

 polar distance. The effect of the demagnetizing corresponds 

 to an equal loss of vis viva, which is the measure of the thermal 

 effect produced ; and this effect is the only one which accompanies 

 the demagnetization." 



The fact that an iron bar is heated by successive magnetiza- 

 tions and demagnetizations has been known for a long time ; but 

 only recently have experiments been made which indicate that 

 this heat is produced at the moment of demagnetization. In a 

 paper " On the Calorific Effects of Magneto-electricity, and on the 

 Mechanical Value of Heat" (Phil. Mag.S.3. vol.xxiii. 1843),Dr. 

 Joule first showed that heat was developed in an iron bar when 

 it was rotated between the poles of a powerful magnet, and also 

 determined that the heat thus produced was proportional to the 

 square of the inductive force. These experiments will ever be 

 regarded with interest; for they led Joule to the first experi- 

 mental determination ever made of " the mechanical value of 

 heat." It may here be of interest to present the following ac- 

 count of the experiments made by Van Breda and Grove, taken 

 from DagmVs Traite de Physique, 1861, vol. iii. p. 621 : — "M. 

 Van Breda having enveloped a tube of iron with a helix through 

 which he passed an intermittent current, found a heating of the 

 iron, due to the alternative displacement of the molecules f, the 

 heat being shown by the dilatation of the air contained in the 

 tube, which formed the reservoir of an air-thermometer. Grove 

 subsequently determined the heating of an armature of soft iron, 

 on passing an intermittent current in the wire of an electro- 

 magnet on which the armature was placed, or in turning near it 



lowing passage in the paper of M. Cazin (Comptes Rendus, vol. lxxv. 

 p. 1266) : — " When we pass an intermittent current in the wire of an elec- 

 tromagnet, the recent experiments of MM. Jamin and Roger have demon- 

 strated in a definite manner that the core is heated." The method by 

 which they discovered this fact is not stated by Cazin. See Ann. de Chim. 

 etde Phys. vol. xvii. (1869). 



* Phil. Mag. Feb. 1873, p. 157- 



f The heat observed, however, may not be entirely due to these motions ; 

 for the thermal effects may in part be due to the currents induced in the 

 iron on magnetization and demagnetization. 



