150 Transactions of the American Institute. 



We hence deduce a general rule of the mutual action of magnets, viz. : 

 Like ends, or poles, repel ; unlike poles attract. 



In the course of the solution of our proposed problem, we will have 

 frequent occasion to use the guidance of the following experimental 

 tests : 



Place the length of the bar you would test at right angles to the 

 length of the needle, and pointing toward its center. If the needle 

 remains at rest the bar is devoid of magnetism. If it rotates, the bar 

 is a magnet ; and if the north end of the needle approach the bar, 

 then the south end of the latter is near the needle ; if the south end 

 of the needle approach the bar, then the north magnetic end of the 

 bar is nearest the needle. 



We have seen that both the lodestone and the great magnet con- 

 ferred their magnetic properties on masses of iron which touched 

 them ; but contact with the magnet is not necessary for calling into 

 existence attractive properties in iron, as you will clearly apprehend 

 from the experiments I am about to make. I again bring over the 

 lens of the lantern a glass plate strewn with iron filings, and I draw 

 through them the end of this short iron rod, while near to the other 

 extremity I hold this lodestone. You observe on the screen the con- 

 siderable distance that separates them ; yet you see the filings adher- 

 ing to the iron, just as they did when the stone was in contact 

 with it. 



A similar experiment I will now perform with the great magnet. 

 My assistants have unscrewed its conical caps, and have brought the 

 two iron cores and the two sets of spools in close contact. Thus we 

 have made out of the two cores one magnetic bar of seven feet in 

 length. I now lower the battery plates, and the iron is powerfully 

 magnetized. My assistant will stand against one of its ends, and I 

 place against his chest this iron rod over four feet long ; the bar is 

 thus separated from the magnet by the thickness of his body, yet on 

 bringing these rings to its free end they cling to it and to each other, 

 and swing like a chain from its extremity ; the bar is magnetized 

 through the body of a man. 



Again, another experiment I will make. I take this bar of soft 

 iron, fourteen inches long and two inches in diameter, and ascending 

 these steps I hold it aloft, a foot or more above the end of the great 

 magnet. My assistant will now hand me those iron rings, each 

 weighing three pounds ; I bring one to the end of the bar nearest the 

 magnet ; the ring firmly adheres to it ; to this ring I suspend another ; 

 and lifting the iron rod yet higher I have attached another, and 



