C. GENERAL PHYSICS. 161 



powerful and accurate hydraulic press, is promised by tlie 

 distinguished author. 6 B, LXXXL, 205. 



ox THE DISTEIBUTION OF MAGNETISJI IN BUNDLES OF INFI- 

 NITELY LONG, VERY THIN PLATES OF IRON. 



M. Jamin states that, having investigated the distribu- 

 tion of magnetism in infinitely long thin plates of iron, and 

 found it represented by an exponential formula whose con- 

 stants depend respectively upon the nature and the temper- 

 ature of the iron or steel, he has now continued his research- 

 es by combining any number whatever of such plates, but 

 still considering the case when the bundle is infinitely long 

 with reference to its other dimensions. The first result of 

 this study was to show that a saturated magnet is a bundle 

 of elementary magnetic threads inclosed within their aver- 

 age section as by a band, but expanded toward their extrem- 

 ities. The number of these bundles depends only upon the 

 mean section, provided that their polar surfaces are suffi- 

 ciently expanded. The total quantity of magnetism in the 

 thin plates is equal to the product of its section by a con- 

 stant factor, which factor is independent of the length, and 

 is proportional to the thickness of the plate, and also to its 

 breadth. These three laws can be easily demonstrated by 

 determining the magnetic force of plates of variable length, 

 breadth, and thickness. By means of a number of bars hav- 

 ino; a length of ten inches and a breadth of less than one 

 inch, Jamin formed a compound magnet whose dimensions 

 were alterable at pleasure ; this has enabled him to test his 

 laws under a very wide range of circumstances, and it was 

 demonstrated thereby that by superposing these plates to 

 any number whatever, and in any direction whatever, the re- 

 sulting bundle always contains the algebraic sum of the mag- 

 netism of the various elements. This, however, is only true 

 Avhen the bundle is so proportioned that its length may be 

 considered as infinite. By combining fifty such plates, as 

 nearly identical as possible, in different manners, he has very 

 successfully investigated the law of distribution of the total 

 magnetism ; and finds that it can be exactly represented by 

 an exponential formula similar to that which he has already 

 shown to obtain in reference to simple plates, and which 

 therefore enables one, knowing certain constants which may 



