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VIII. — Experimental Researches on the Lifting Power of the Electro-Magnet. 

 Part III. By the Rev. T. R. Robinson, D.D., M.R.I. A., F. R.S., (^c, c^-c. 



Read November 30, 1857. 



In my last paper on this subject I showed that with a given exciting power, 

 measured by -^ = product of the current into the number of spires, the lifting 

 force of an electro-magnet is less as the spires are distributed over a greater 

 length of its arms. Thus, it is seen from Tables xii. and xiii., that, with the 

 same -^j the forces are 911 and 733, the lengths covered being 2'-l and 10''1 

 for each arm. I showed that part of this diminution arises from the oblique 

 action of the remote spires, and part from the imperfect transmission of the 

 magnetism in their immediate vicinity. These, however, account for but a 

 small portion of it, as is evident if the spires be kept in a given position while 

 the length is varied. 



It is the object of this paper to detail a series of such experiments, which 

 were instituted in hopes of finding some simple relation between the position 

 of the spires, the length of the magnetic circuit, and the lifting force L : and 

 though I have been disappointed in this, yet I think the results are not without 

 value, both as pointing out some of the influences which complicate the prob- 

 lem, and as affording numerical data by which theoretical investigations on this 

 subject may be tested. 



Before describing them, it may be useful to state what appears to be the 

 action occurring in an electro-magnet, which is excited while its poles are con- 

 nected. As far as I know, this case has not been examined by theory, and is 

 quite different from that of a permanent magnet, whose poles are unconnected. 

 There, free magnetism, which is null at the centre, increases with opposite po- 



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