demonstrating a limit to the Magnetizahility of Iron. 313 



tricity in the first table should have the same effect as eight in 

 the second table. The difference, if any, should be due to the 

 increased length of the iron. I do not think myself justified 

 in assigning any amount to this difference, which, however, seems 

 to increase in value as the section of the magnets decreases. In 

 order to determine this and many other circumstances of great 

 interest, it would be necessary to conduct experiments in a much 

 more comprehensive manner, and to examine more minutely into 

 the various powers of hard and soft iron and iron wire. 



I think, however, that I have by these experiments discovered 

 a most important law, namely. The attractive force of the electro- 

 magnet is directly as the square of the electric force to which its 

 iron is exposed; or if E denote the quantity of electricity, M the 

 magnetic attraction, and W the length of wire, M = E^W^*. 



It must be confessed that there are many instances in the 

 above tables which seem to form exceptions to this law. I con- 

 sider, however, that the effects of magnetic inertia, and sources 

 of error which I have found it impossible to avoid, are sufficient 

 to account for these. Perhaps the fairest way of comparing the 

 law with experiment is, to take the' mean of the magnetic attrac- 

 tions of all the magnets in the first table, and the mean of 

 Nos. VIII., IX. and X. in the second table, omitting Nos. VI. 

 and VII., because it is clear that they are at last becoming satu- 

 rated with magnetism. The means of the attractions observed^ 

 and the estimated results, are tabulated below. h i 



iw 



Anxious to ascertain whether the law obtained in lifting as 

 well as in distant attraction, I made the following rough experi- 

 ment with a horse-shoe electro-magnet made of a cylinder of iron, 

 7 inches long, and f ths of an inch in diameter, and wound with 

 five yards of thick copper wire. The law seems in this case to 

 fail principally because the iron is sooner saturated with mag- 

 netism ; hence the propriety of making electro-magnets for lift- 

 ing of considerable bulk, 



* Jacobi and Lenz communicated their report on magnetic attraction to 

 the Academy of St. Petersburgh in March 1839, or two months previously 

 to the date of this paper. In it they announced a law similar to the above. 

 —Note, May lB5^jioi.-v7i©w;^iettfwM rfJ^oai .3ii^ aoniit ^siri) Mir 



