814 Mr. J, P. Joule on t/ie Magnetizability of Iron, 



I remain, dear Sir^ 



Yours truly, 



J. P. Joule, 



Broom Hill, near Manchestel*, 

 Dear Sir, July 10, 1839. 



The following experiments were designed to test still further 

 the law given in my last letter. Two pairs of electro-magnets 

 were constructed ; each of the first pair was made of a bar of 

 iron, 30 inches long and 1 inch square ; each of the second pair 

 was made of a bar of iron 30 inches long, 2 inches broad, and 1 

 inch thick. The shai*p edges were ground down to prevent in- 

 convenience in the v/inding of the wire. Each magnet was pro- 

 perly insulated, and wound with eighty-eight yards of covered 

 copper wire y^th of an inch in diameter. 



The attractions were measured in precisely the same manner 

 as before, saving the substitution of copper for wood, to keep the 

 magnets at the proper distance apart. The attraction of the 

 suspended magnet for the fixed one was measured in ounces 

 avoirdupois. 



The experimental results arc quite as near to the theoretical 

 as could be expected, considering the several sources of eiTor* 

 Those belonging to the first pair are particularly satisfactory^ 



