194 Prof. Thomson on the Mechanical Values of 



as an experimental result, to be simply proportional to the quan- 

 tity. Mr. Joule's result has been veritiecl by independent expe- 

 rimenters in France, Italy and Germany. The author pointed 

 out other applications of his investigation, some of a practical 

 kind, and others in the Mathematical Theory of Electricity. He 

 mentioned, that although he had first arrived at the results in 

 1845, and used them in papers published in that year, the first 

 explicit publication of the theorem regarding the mechanical 

 value of the electrification of a conductor appears to be in 1847, 

 in a paper entitled " Ueber die Erhaltung der Kraft," by 

 Helmholtz. 



II. Magnetism. 



If a piece of soft iron be allowed to approach a magnet very 

 slowly from a distant position, and be afterwards drawn away so 

 rapidly, that, at the instant when it reaches its primitive position, 

 where it is left at rest, it retains as yet sensibly unimpaired the 

 magnetization it had acquired at the nearest position, a certain 

 amount of work must have been finally expanded on the motion 

 of the iron. For during the approach, the iron has only the 

 magnetization due to the action of the magnet on it in its actual 

 position at each instant; but at each instant of the time in which 

 the iron is being drawn away, it has the whole magnetization 

 due to the action of the magnet on it when it was at the nearest. 

 Hence it is drawn away against more powerful forces of attrac- 

 tion by the magnet than those with which the magnet attracts 

 it during its approach ; from which it follows, that more work 

 is spent in drawing the iron away than had been gained in letting 

 it approach the magnet. The sole efiect due to this excess of 

 work is the magnetization which the iron carries away with it ; 

 and consequently, the mechanical value of this magnetization 

 must be precisely equal to the mechanical value of the balance 

 of work spent in producing it. 



After a very short time has elapsed with the piece of soft iron 

 at a great distance from the magnet, it will have lost, as is well 

 known, all or nearly all the magnetization which it had acquired 

 temporarily in the neighbourhood of the magnet ; and in this 

 short time some energy, equivalent to that of the magnetization 

 lost, must have been produced. Mr. Joule's experiments show 

 that this energy consists of heat, which is generated in the iron 

 during demagnetization ; and we infer the remarkable conclusion, 

 that at the end of the process which has been described, or of 

 any motion of a piece of soft iron in the neighbourhood of a 

 magnet, from a certain position and back to the same, the iron 

 will be as much wanner than it was at the beginning, as it would 

 have been without any magnetic action, if it had received the 



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