Magnetic Induction in Straight Iron Rods. 



487 



Fig. 3. 















































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Field in C.G.S. units. Curves 1 & 2, 



2 units ; No. 11, i cm. =5 units. 



The first equation was given by Biot in his Trait e dc Phy- 

 sique, vol. iii. page 77 (1816). He obtained it by comparing a 

 magnet to a dry electric pile. Green in his Essay derived a 

 formula for the case of a long rod placed in a magnetic field 

 parallel to the lines of force. His formula gives the same distri- 

 bution as that of Biot. Prof. Rowland, who has carried out a 

 great many experiments on the distribution of magnetism, has 



