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On Magnetic Circuits in Electric Machines. [Mar. 18, 



been left for some time with its poles uncovered had less residual 

 magnetism after a momentary current had passed through its helices, 

 than another magnet which had been in active closed circuit, even if 

 the battery current had, in the latter case, to overcome a considerable 

 amount of residual magnetism. 



We found, moreover, that soft iron magnets retain their residual 

 magnetism longer, and are capable of acquiring increased magnetiza- 

 tion much more rapidly after having been bearing weights (thereby 

 keeping the iron in a state of strain), than if they have been left in 

 their normal condition and without bearing any weight at all. 



The conditions under which the closed magnetic circuit retains its 

 force are not yet clearly established. 



With the 58 lb. magnet a succession of gentle taps struck vertically 

 with a wooden mallet upon the centre of the armature, while resting 

 on the magnet in closed circuit, in a very few moments completely dis- 

 sipated the magnetic force so far as the sustaining power of the magnet 

 was concerned. 



Removal of any portion of the weight suspended to the armature of 

 a magnet hung up in closed circuit likewise tends to dissipate the 

 force of the circuit. For example : — Half an hour after the removal of 

 a weight of 10 lbs., which had been suspended to the armature of a 

 U magnet for twenty-one days, the armature fell off on receiving a 

 slight touch. In another experiment, a \J magnet, which, was capable 

 of sustaining 7 lbs., and which had actually been suspending 4 lbs., 

 was left for two months with the armature on only, the weight having 

 been removed ; at the end of that time a very slight shake was suffi- 

 cient to cause the armature to fall off. Many other examples might be 

 quoted to show that release from strain diminishes the magnetic force 

 of the circuit. 



In these experiments, in which the closed magnetic circuits had 

 given way, the soft iron had been in a state of strain from which it 

 had been released by the removal of the suspended weights. But 

 when no weights were hung upon the armature, and the iron had 

 never been in a state of magnetic tension, the closed magnetic circuit, 

 so far from diminishing, increased in force. The 58 lb. magnet was 

 excited with a voltaic current so feeble, that although the magnet 

 could be lifted by the armature in closed circuit, yet great care was 

 necessary that the lift should be exactly vertical ; and very little force 

 was required to slide the armature off the poles. After the lapse of a 

 month the armature was so firmly held that the utmost exertion of 

 manual force could not stir it by a sliding movement, and the whole 

 magnet could be raised from the ground even if tilted as much as 15° 

 from the perpendicular. 



The magnetism of the closed circuit of the 58 lb. magnet disappears 

 after repeated up ar.d down movements of either one or both of its 



