Lifting Power of the Electro-Magnet. 531 



effect produced is less than would be caused by adding it to another current 

 capable of producing the force A. In this state all the molecules in the circuit 

 must be equally polarized, for A is the same when only one arm has been ex- 

 cited. It occurs, though but feebly, even when the keeper is separated from 

 the poles by a very small interval. 



When the keeper is raised, and the force 2 is destroyed, a further decrease 

 of the force takes place, till the coercive force 3 balances the attraction of the 

 polarities. This is the case of residual or permanent magnetism, which I de- 

 scribe by the force \ ; it is very powerful in hard steel, but is found in all iron 

 however soft. By repeated excitation it rises to a maximum, and then is not 

 entirely removed by reversing the current, unless that be able to produce half 

 the maximum. If the magnet be hard steel, it is able to overpower a reverse 

 excitation, whose L would of itself considerably exceed it ; and if destroyed it 

 cannot be restored, but by one whose L is four or five times its own amount. 



An electro-magnet, like a permanent one, bears a greater load, if this be 

 gradually increased. 



It requires a considerable time to attain its full power, not less, in any of the 

 magnets which I have used, than ten minutes. 



It becomes less powerful by long-continued magnetization in one direction. 

 This is connected with its acquiring permanent magnetism, and the reverse cur- 

 rent then produces less efi'ect than the direct, unless it can produce half the 

 maximum. That current is, therefore, the best where the polarity is to reci- 

 procate, as in electro-magnetic engines. 



It is affected by a rise of temperature. K of soft iron, its power decreases, 

 though irregularly, having minima and maxima between 32° and 220°. The 

 coefficients of the interpolation formula which represent the change, vary with 

 the diameter and length of the magnet. 



In one of hard steel the power increases with the temperature from 50° to 

 190° ; indeed, probably until the heat is sufficient to lower its temper. The 

 amount of change is four times that of iron of the same dimensions. 



In one whose keeper and base are iron, but the parts covered by the helices 

 steel, there is an increase almost exactly equal to the decrease in the iron one. 



The central part of the magnet is probably inactive. 



The force at the pole of an electro-magnet is the resultant of four others 



