308 



Mr. J, P. Joule's Account of Expmiments 



ments gave 162 i-evolutions per minute with the firsthand 130 

 with the second magnet* ,>ff wii-M;frti?^ infU^ib r..? ^nfltd io ^.>. ,;. 

 In the further prosecution of my inquiries, I took fidx pieces 

 of round iron of different diameters and lengths, and also a piece 

 of hollow round iron, half an inch in diameter, and yV^h of an 

 inch thick in metal ; these were bent into the U-fonn, so that 

 the shortest distance between the poles of each was half an inch ; 

 each was then wound (with the usual precautions to ensure insu- 

 lation) with ten feet of covered copper wire of ^^^th inch dia- 

 meter. The lengths and diameters are given in the following 

 table. No. 1 is the hollow magnet. The attraction was ascer- 

 tained by suspending a straight steel magnet, 1^ inch in length, 

 horizontally to the beam of a balance, and bringing the several 

 electro-magnets directly underneath at the distance of half an 

 inch, which was preserved by the interposition of a piece of wood 

 half an inch thick. Care was taken that the battery remained 

 constant during the experiments. 



No. 1. 



No. 2. 



No. 3. 



No. 4. 



No. 5. 



No. 



No. 7. 



Length in inches 



Diameter in inches 



Weight lifted in ounces 

 Attraction for steel 1 

 magnet in grains... J 



6 



36 

 7-5 



5i 



52 

 6-3 



2# 



92 

 51 



5^ 



a 



3^6 

 50 



a 



5^2 

 41 



k 



20 



4-8 



3-6 



,urA steel magnet of such dimensions as enabled me to compare 

 it fairly with the electro-magnets, was found to exert an attrac- 

 tion of 23 grains for the small steel magnet, though its lifting 

 power was only 60 oz. 



These results will not appear surprising if we consider, first, 

 the resistance which iron presents to the induction of magnetism ; 

 and secondly, how very much the power of iron to conduct mag- 

 netism is exalted merely by the completion of the ferruginous 

 circuit. In order, however, to explain why the long electro- 

 magnets have a greater attracting power at a distance, though 

 they lift less weight, than the short magnets of the same dia- 

 meter, it will be necessary to observe that it was impossible to 

 wrap the whole ten feet of wire on the smaller magnets, without 

 disposing it in two or three layers (according to the size of the 

 magnets). This was a great disadvantage ; and one might have 

 anticipated in consequence, that the power of the long magnets 

 would be greater than that of the short ones for lifting, as well 

 as distant attraction, which is contrary to the results of the 

 table ; this may however be explained, if we admit that the com- 

 parative resistance of the iron of the electro-magnet increases to 

 a very great amount, when its magnetism is so greatly excited 

 as by the contact of the armature. 



