34 Mr. J. P. Joule's Account of Experiments 



Experiment II. — Having provided a pair of tapered poles ter- 

 minating: in vertical edges, 1 inch long and ith of an inch in 

 breadth, i caused them to be slid on the poles of the electro- 

 magnet until within 1 \ inch from each other. A cylindrical 

 bar of bismuth 1^ inch long, A of an inch in diameter, and 

 weighing 174 grains, was suspended by a filament of silk from 

 a proper support," so as to vibrate between the tapered poles. 

 The average numbers of vibrations in each minute of time through 

 the quadrant of a circle were then found to be — 



with 1 cell in a series of 1 . . . 44, vibrations. 

 ... 4 cells ... 2 ... 91 



... 16 ... 4 . . . 17 



The cm-rents being as 1, 2 and 4, and the vibrations 44, 9| and 

 17, or nearly in the same ratio, it follows that the repulsive action 

 of the magnetic poles was as the square of the current, and con- 

 sequently that the diamagnetism of the bismuth is a quality not 

 self-inherent, but induced by the magnetic action to which it is 

 exposed. I am happy to have been thus enabled to confirm the 

 important fact, discovered by M. Ed. Becquerel and Dr. Tyndall, 

 by experiments made without any knowledge* of the researches 

 they were conducting almost simultaneously on the same subject. 



Experiment III. — The tapered poles remaining at \\ inch 

 asunder, I suspended a piece of soft iron, 3 inches long, 1 inch 

 deep, and jth of an inch thick, at the distance of a quarter of an 

 inch above the poles. Using one cell of the battery, this small 

 piece of iron was attracted with a force of 6f oz. ; but with 16 

 cells in a series of 4, with a force of no less than 1\\ oz. In 

 this instance we notice a slight falling away from the theoretical 

 attraction, owing no doubt to the gradual approach of the limit 

 to magnetizability in the small bar of iron. 



Experiment IV. — The tapered poles having been removed, a 

 flat bar of soft iron, 14 inches long, 3 inches in breadth, and 

 1 inch thick, was placed at various distances from the poles of the 

 electro-magnet, and the attractions measured as follows : — 



\ in. dist. 



with 1 cell 102 



with 16 cells in a series of 4 976 



Here, again, we have evidences of an approach towards the limit 



* The electro-magnet with which the above experiments had been made 

 was sent to the Exhibition of Industry in the middle of February. M. Bec- 

 aaerel's paper was published in the Annates de Chimie for May, Dr. Tyn- 

 dall's in this Magazine for September, after having been previously com- 

 municated to the Ipswich Meeting of the British Association. 



