with a powerful Electro-magnet. 35 



of niagnetizability, for the attractions with a current of 4 are 

 only ten times, instead of sixteen times as great as those ob- 

 served with a current of 1. 



The electro-magnet I described some years ago* consisted of 

 a core of iron, half an inch thick, enveloped by a coil of wires 

 weighing 60 lbs. With a battery of ten cells, similar to those 

 employed in the present experiments, a bar of iron 3 inches 

 broad and | an inch thick, was attracted at the distance of \ of 

 an inch with a force of 480 oz., at A an inch with a force of 168 

 oz., and at 1 inch with a force of 77 oz. Both electro-magnets 

 having been constructed on the same principle, their attractive 

 powers ought to be proportional to the weight of coil and number 

 of cells, and therefore to be represented by 60x10 = 600, and 

 100 x 16 = 1600. As this is tolerably well borne out by com- 

 paring the actual results of the above experiments, we may infer 

 that little or no advantage was obtained by increasing the thick- 

 ness of the core of iron from half an inch to one inch. 



Experiment V. — A flat bar of iron, If inch deep and |-th of 

 an inch thick, being placed with its thin edge in contact with 

 the poles of the electro-magnet, the following weights had to be 

 applied in order to overcome the attraction in contact : — 



With 1 cell in a series of 1 . . 64 lbs. 

 ... 4 cells ... 2 . . 72 ... 



... 16 ... 4 . . 96 ... 



But when the bar of iron used in Experiment IV. was placed in 

 contact with the poles, so as just to leave a quarter of an inch in 

 breadth for the place of contact of the flat bar, the attraction of 

 the latter with 16 cells was found to be only 82 lbs. Thus it 

 would appear that 14 lbs. out of the 96 lbs. in the previous ex- 

 periment were owing to the distant attraction of that part of the 

 poles not in contact with the bar. We may therefore conclude, 

 that while the attraction in contact, using one cell, was 64 lbs., 

 minus say 1 lb. for distant attraction, that produced by a cur- 

 rent four times as great was only increased to 82 lbs. And it 

 must be remarked, that the greater part of this small increase was 

 doubtless owing to the action of the broader part of the iron core 

 which still remained unneutralized. It would therefore appear, 

 that the greatest observed attraction in contact was, in this electro- 

 magnet, about 70 x 5 = 350 lbs. per square inch of the surface 

 of each pole, or otherwise that the greatest magnetic attraction 

 of one square inch of surface for another square inch was 175 lbs. 

 Several years ago 1 gave 140 lbs. as the apparent limit of attrac- 

 tion in contactf- The force of current employed in obtaining 



* Philosophical Magazine, S. .'J, vol. xxiii. p. 268. 

 t Ibid. S. 4. vol. ii. p. 468. 

 1) 2 



