CLOSED ELECTRO-MAGNETS. 



299 



in the case where both are in contact, is proportional to the 

 strength of the magnet*/^ For by Tyndall the strength of the 

 magnet is regarded as proportional to the strength of the galvanic 

 current, and certainly not without reason,as the reaction of a small 

 sphere of iron, which this skilful investigator always makes use 

 of, must be very inconsiderable. The magnetic strength, how- 

 ever, of which the author speaks, is that which is in part due to 

 the reaction of the submagnet, and which is not proportional 

 to the strength of the current, and which depends in a great 

 measure upon the nature and dimensions of the submagnet. 

 But in both cases it would be the true magnetic force, which, 

 in apparent opposition to the principle of mechanics, is simply 

 and directly proportional to the attraction. 



The author in the same paper treats of the residual lifting power 

 of closed electro-magnets, and gives measurements, from which 

 it appears that this is not constant, but increases with the 

 strength of the current, although in a less ratio than the total 

 powder. To ascertain how far this residual power depends upon 

 the coercive force of the metal, he determined it with magnets of 

 unhardened steel and with those of soft iron of the same dimen- 

 sions, both being closed by the same submagnet constructed 

 of the material last-mentioned. The following table will give 

 a view of the results : — 



should, by increasing the current, soon find the attraction at a distance to be 

 greater than in the case of contact." I record the objection as it appears to be 

 a valid one, reserving its closer examination for another opportunity. — J. T. 

 * Pogg. Ann. vol. Ixxxiii. p. 1. Phil. Mag. [iv.] vol. i. p. 265. 



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