202 Mr. W. Sturgeon om Electro-Magticis. 



With a new battery, and a solution of salt and water at a 

 temperature of 190° Fahr,, the small electro-magnet supports 

 16 pounds. 



I noticed in some of my earliest experiments, that a bar of 

 soft iron which had been intensely magnetized by galvanic 

 action, retained a considerable degree of polarity when the 

 exciting cause had been long removed, — a phaenomenon now 

 more conspicuously displayed by the employment of larger 

 masses ; nor does it appear to be an easy matter to subdue, 

 entirely, tliis residual polarity. The poles may be reversed 

 as frequently as we please ; but still some, and frequently a 

 considerable degree of polarity remains unneutralized. If the 

 cross piece be permittetl to remain attached to the poles when 

 the galvanic connection is broken, the residual polarity will 

 still keep them together with an astonishing force; as is mani- 

 fest by the very great weight which the magnet continues to 

 support before the cross piece is disengaged from its poles. A 

 residual polarity, however, still remains; but so enfeebled are 

 its energies by the slightest interruption of polar contact, that 

 the cross piece will seldom be supported a second time with- 

 out a renewed excitation by the battery. 



The vigorous residuum of polarity which retains the cross 

 piece to the magnet arises from a continued mutual attraction 

 between the two ; for whilst the batter}' operates upon the 

 magnet, and excites it to action, the latter in its turn also 

 excites the magnetism of the iron connecting its poles, which 

 iron becomes as deci(!edly polar as the magnet itself; a 

 7!0)ih 2^ole being determined in that end of it which is in con- 

 nection with the south pole of the magnet, and a south pole 

 in the other end, which is connected with the north pole of 

 the magnet. The four magnetic poles thus brought into play, 

 and in vigorous operation on each other, will, if not separated, 

 retain their ])ositions at the points of connection, even though 

 the first exciting cause be entirely withdrawn. But if the 

 connection of the two pieces of iron be in the least interrupted, 

 their magnetism immediately recedes from the extremities, and 

 becomes equally distributed in the metal; the vigour of the 

 poles vanishes, and their magnetism becomes totally incapable 

 of keeping them attached to each other. 



Precisely the same kind of reasoning will explain the cause 

 of that well known deterioration of magnetic force which inva- 

 riably takes place by removing the cross piece from a highly 

 excited steel magnet. The excitation in this case, whether it 

 be performed by the operation of a magnet, or by the more 

 gradual process of adding small weights to those already sus- 

 pended, is carried on whilst the poles and the cross piece are 

 in contact ; and both become excited at the same time. The 



magnet 



