82 The Rev. Prof. Ritchie on certain Differences 



electro-magnets which I formerly made public in your Journal*, 

 I take the liberty of sending you the present investigation, which 

 may be regarded as the completion of my former paper. 



Experiment 1 . Suspend a piece of soft iron, C D, at the 

 extremity of a slender delicate balance of light wood ; place 

 a permanent horse-shoe magnet below it, and ascertain its at- 

 tractive force, by weights put into the scale G, when it is in 

 contact, and also when it is removed to different distances from 

 the soft iron. Remove the permanent magnet, and substitute a 

 very sJwrt electro-magnet of equal lifting power. Remove it 

 to the same distances as before, and the attractive power will 

 diminish very rapidly compared with that of the permanent 

 steel magnet. 



Exp. 2. Instead of the short electro- magnet, substitute a' 

 very lono- one (one of two or three feet long, for example,) and 

 of equal carrying power ; remove it to the same distances and 

 ascertain its attractive power, and it will be found that its at- 

 traction for the lifter at these distances will 7wt diminish so' 

 rapidly as that of the short one. The longer the electro-mag<-. 

 net becomes, the more does it approach to the character of the 

 permanent magnet in all its properties. 



Exp. 3. Instead of making the electro-magnet of soft iron, 

 make it of hard iron or untempered steel ; repeat the preceding: 

 experiments, and its attractive power at a distance compared; 

 with its lifting power will be much greater than in the case of 

 the electro-magnet of soft iron. 



These facts, which, as far as I know, have not before been 

 published, will enable us to account for this property on prin- 

 ciples previously recognised. The perfect equality of action 

 and reaction must be found to exist in this case as well as in 

 every other in which_ybrc<? of any kind is concerned. The elec- 

 ti-icity which has been decomposed and arranged in the soft 



* [Prof. Ritchie's papers " On the Power of an Electro-Magnet to retain 

 Us Mannctism after the Battery has been removed,'^ and " On certain curious 

 PropeHics of Conmon and Electro-Magnets," will be found in Lend, and 

 Edinb.Phil. Mag., vol.iii. pp. 122, 124.— Edit.] 



