294 Mr HARRIS'S Experimental Inquiries concerning 



ed all the effect which the bar of itself could otherwise produce ; 

 so that the bar being, as it were, insulated by the intervening 

 iron, the final force of attraction might be considered to depend 

 exclusively on the iron. 



27. Although the distant poles of magnets by induction 

 evince an attractive force inversely proportional to the length of 

 the iron ; yet the pole immediately opposed to the inductive 

 magnet would seem to possess the same force in all cases, with- 

 out any relation to the length of the iron ; since by substituting 

 a small magnet a, Fig. 1, for the cylinder of soft iron, and placing 

 immediately under it in succession, at a constant distance, masses 

 of iron of different lengths, the force of attraction indicated on 

 the arc was observed to be in each case the same. The force, 

 therefore, induced in each mass of iron must have been alike, 

 since the total attractive force, as will be further shewn (37), is 

 observed to vary with the force induced in the iron ; the power 

 of the magnet remaining unchanged, and all other things re- 

 maining the same. 



This result is quite consistent with the general effect observed 

 in opposing a long mass of iron to the pole of a magnet, in which 

 case the distant extremity of the iron does not appear, except 

 by very delicate tests, to be at all magnetic ; whilst shorter 

 lengths, as already shewn (19, 20), exert a considerable attrac- 

 tive force *. 



* It may be from this circumstance that some profound investigators of magne- 

 tic phenomena have found, that a hollow sphere of iron exerted as much effect on a 

 compass needle as a solid mass of the same dimensions ; which might be reasonably 

 supposed to be the case, as the iron could only become magnetic by induction, in which 

 case the force of the proximate poles would be always the same. The force which 

 such ball or shell, however, could exert on some third mass, not previously magne- 

 tic, would probably be found to be very different. 



