demonstrating a limit to the Magnetizahility of Iron, 451 



A third electro-magnet, No. 3, was made of a piece of iron 

 0*7 of an inch long, 0*37 of an inch broad, and 0*15 of an inch 

 thick, of which the lateral edges were well-rounded. It was bent 

 into the semicircular shape, and covered with 19 inches of insu- 

 lated copper wire ^\jth of an inch in diameter. 



Anxious to procure a still greater variety, I made what might, 

 from its extreme minuteness, be almost termed an elementary 

 electro-magnet. It was the smallest, I believe, hitherto made; 

 and was constructed of a piece of iron wire \ of an inch long 

 and ^jih. of an inch in diameter. It was bent into a semicircle, 

 and wound with three turns of uninsulated copper wire ^^^th of 

 an inch in diameter. 



In the following tables, in which the experiments with the above 

 electro-magnets are recorded, the first column gives the quantity 

 of electrical current in degrees ; the second contains the same 

 multiplied by the length of the coils in feet ; and the last con- 

 tains the lifting power in pounds avoirdupois. 



Table I. — Electro-magnet No. 1. Weight of its iron 15 lbs. 

 Length of coils 23 feet. 



Subsequently, with a more powerful battery, the weight ne- 

 cessary to remove the keeper was 2090 lbs., which is, I believe, 

 a greater weight than any magnet has hitherto carried, and is 

 certainly vastly superior to the performance of any other of the 

 same weight ; but I can show that this power, great as it is, is 

 not so much as is due to the form I have employed. 



The latter part of the above table was obtained experimentally 

 before the first part, and in the mean time the proper insulation 

 of the coils from the iron was destroyed by accident ; and not 

 having had the opportunity of refitting the electro -magnet, I 

 have been obliged to supply the column of corrected electric 

 forces, calculated from the power obtained when the insulation 

 was good. 



