Lifting Power of the Electro-Magnet. 309 



Now there is good reason to believe that C has a limit, or, in other words, that 

 the molecules of iron can oppose only a limited resistance to induction ; D and 

 M are as L, and therefore if H be infinite, so must the latter also be, unless M 

 too have a physical limit. What that limit is, — whether the expansion of the 

 hypothetic fluid, or the impossibility of exciting vibratory movement beyond a 

 certain extent, — I do not pretend to determine. For this magnet I believe the 

 A to be under 1000 lbs. Secondly, I would call attention to some other facts 

 that seem important. At No. 26 is found 4'44 for L when there is no exciting 

 force, it is the permanent magnetism A. If we apply a direct current 0'04, it 

 adds to this 9-74 ; if a reverse, it only subtracts 2-25 ; L therefore is a function 

 of \ as well as ofi^. AtNos. 24 and 28 the differences are + 50-17, - 7-25 ; at 

 23 and 29 still wider asunder, after which they begin to approach, but are not 

 the same exactly till F passes ± 0-7. At F — 1'24, the direct and reverse re- 

 sults are identical. It follows from this that the coercive force consists of at 

 least two terms, one changing sign with F, the other depending on the habi- 

 tual direction of the excitation ; the latter is not overpowered completely until 

 L — \A, and does not vanish even when L= 0, as is manifest from Nos. 27 

 and 28. As a practical deduction, we may infer that in all machines involving 

 the reversion of an electro-magnet's polarity, its excitation should not fall short 

 of this. On the other hand, it should not much exceed it ; for the increase of 

 power gained by a given increase of current is constantly lessening, and the 

 consumption of materials augments even faster than the current. 



The seventh column gives the value of \ the residual magnetism, which, for 

 reasons already stated, should be known, in order to compare L with any for- 

 mula. These numbers are given by from six to ten observations ; and it will 

 be observed that they do not vary much. When the reverse happens (as in 



Nos. 4-7) it is evident from the irregularity of the quantity -r^, that the values 



of L are discordant from the rest of the series. It will also be noticed, in con- 

 firmation of what was stated as to the coercive force, that in No. 28 \ remains 

 positive, although diminished, although L is negative. The highest value of it 

 which I ever got is 8'88, which (the keeper being removed in the mean time) 

 gradually decreased till, after 36 hours, it was permanent at 3'17. 



The last column gives A, the residual excitation, or the force which the 



2s2 



