38 Scientific Proceedings, Royal Dublin Society. 



include them merely to show that the extra-polar field was not overlooked ; 

 but we concern ourselves at present with the inter-polar space. 



Want of Symmetry. 

 In our earlier investigation we were much troubled by a lack of symmetry 

 in our diagrams. To facilitate observation, we had erected our apparatus 

 without reference to its magnetic bearing, judging that the earth's field would 

 not appreciably interfere with the strong electro-magnetic field. We were 

 speedily undeceived, however, as in the outer and weak parts of the field 

 distortion was considerable. On swinging the magnet so that its axis lay 

 north and south, matters were improved, but distortion was still present. 



--. &" v ;: :"•*'.'.. 



Fig. 3. 

 We found that the base-plate of our magnet was of rough cast-iron 

 considerably harder than the wrought-iron cores, and having been once 

 magnetized retained that magnetism with remarkable tenacity. Eeversing 

 the current in the coils of the magnet only produced a temporary reversal of 

 the lines in the sole, which assumed its original magnetic state when the 

 current was broken. It was evident then that a certain minimum of 

 current was required to magnetize the sole in a contrary direction to its 

 permanent magnetism. Now, as we were accustomed to regulate the 

 strength of our field by varying the current between *6 and '02 amps., at a 



