300 



Miscellaneous. 



inches in length, and of different diameters, from three inches down to 

 one-third of an inch, to the action of a voltaic current of the same force, 

 in each case, and we obtained the amount of magnetic force represented 

 in the following table. : — 



Diameter of the 

 rods. 



Magnetism observed. 



Magnetism calculated. 



3 



447 



442 





378 



376 



2 



308 



310 



i* 



246 



244 



1 



175 



178 



5 

 6 



158 



156 



i 

 3 



142 



135 



1 



2 



112 



113 



2 

 3 



87 



91 



"This calculation has been made according to the formula m = 131.75 

 d+46.75, in which the constants have been obtained by the method of 

 the least squares. The differences between calculation and observation, 

 are not so large that they cannot be attributed to the inevitable errors of 

 observation, and to circumstances inherent in the qualities of iron, &c- 

 A similar agreement is found between other observations, which we 

 shall describe in the report itself. I think, therefore, we may admit the 

 following law, namely, that the amount of magnetism received by different 

 iron rods of the same length, and submitted to the influence of a current of the 

 same force, is proportional to the diameter of the rods. I must remark, that 

 the constant which we have added in the formula depends upon the 

 magnetic influence which the helix exercises, independently of the 

 nucleus of iron which it incloses. The practical consequences which 

 may be deduced from this remarkable law are of considerable importance. 

 Among these, however, I will at present mention only the following. 

 Having found that the amount of magnetism is proportional to the sur- 

 face of the malleable iron, and taking into account the quantity of iron 

 employed in the electro-magnets, it is ascertained that it is more advan- 

 tageous to employ in the construction of electro-magnetic machines, 

 rods of small instead of large dimensions ; or rather hollow iron, in 

 accordance with my own experiments of 1837, which are found in 

 « Taylor's Scientific Memoirs,' vol. ii. &c. I cannot pass over in silence 

 the experiments of Prof. Barlow, who, as is well known, proved a long 

 time before that the induction of the terrestrial magnetism upon malleable 

 iron,depends only upon the surfaces, and is almost independent of the thick- 



