of Magneto-Electricity. 355 



The results of the preceding experiments are collected in 



Table III. 



the following table : — 



It was discovered by Prof. Jacobi, and by myself also one or 

 two months afterwards*, that the attraction of electro-magnets, 

 either towards one another or for their armatures, is (below 

 the point of saturation) proportional to the square of the 

 electric force. The magnetism in an electro-magnet is there- 

 fore simply as the electric force. Consequently the numbers 

 in column 2 are proportional to the magnetic virtue of the 

 electro-magnet. But on comparing columns 3 and 4 together, 

 it will be seen that the heat evolved is as the square of the 

 electricity. Therefore the heat evolved by a revolving bar of 

 iron is proportional to the square of the magnetic influence to 

 which it is exposed. 



After the preceding experiments there can be no doubt that 

 heat would be evolved by the rotation of non-magnetic sub- 

 stances, in proportion to their conducting power, Dr. Faraday 

 having proved the existence of currents in such circumstances, 

 and that their quantity is proportional, ceteris paribus, to the 

 conducting power of the body in which they are excited. I 

 have not made any experiments on this subject, but in the 

 next part we shall have occasion to avail ourselves of the good 

 conducting power of copper, in conjunction with the magnetic 

 virtue of the bar of iron, in order to obtain a maximum result 

 from the revolution of a metallic bar. 



[To be continued.!] 



* Annals of Electricity, vol. iv. p. 131 . Jacobi and Lenz communicated 

 their report to the St. Petersburgh Academy in March 1839, — two months 

 previous to the date of my paper. 



[f Mr. Joule requests us to insert the following erratum in the former 

 portion of his paper : p. 27 4, for B C, C D, &c, read B C, DE, &c— Edit.] 



2A2 



