Electro-Magnet. 203 



other battery was then substituted for the above, contauiing about 

 three times the same quantity of zinc surface ; with this, at the first 

 instant of immersion, the magnet sustained 1600 lbs. ; after the acid 

 was removed, it continued to support, for a few minutes, 450 lbs. ; 

 and in one experiment, three days after the battery had been excited, 

 more than 150 lbs. were added to the armature* before it fell. It was 

 evident from these experiments, that this magnet required a consid- 

 erably larger quantity of zinc surface in proportion to its weight, to 

 magnetize it to saturation, than that described in the former paper. 

 Accordingly the two batteries, before mentioned as containing 4^ 

 square feet, were prepared. With one of them, at the first immer- 

 sion, the magnet readily supported 2000 lbs. A sliding weight was 

 then attached to the bar ; the battery was suffered to become per- 

 fectly dry, and on immersing it again, the magnet supported 20G3 

 lbs. The effect of a larger battery was not tried. 



To test its power of inducing magnetism on soft iron, two pieces 

 of round iron li inches in diameter and 12 inches long, were inter- 

 posed between the extremities of the magnet and the armature — 

 with this arrangement, when one of the batteries was immersed, the 

 pieces of iron became so powerfully magnetic as to support 155 lbs. 



To exhibit the effects produced by instantaneously reversing the 

 poles, the armature was loaded with 56 lbs. vi^hich added to its own 

 weight made 89 lbs. ; one of the batteries was then dipped into the 

 acid and immediately withdrawn, when the weight of course continued 

 to adhere to the magnet ; the other battery was then suddenly im- 

 mersed, when the poles were changed so instantaneously, that the 

 weight did not fall. That the poles were actually reversed in this 

 experiment, was clearly shown by a change in the position of a largo 

 needle placed at a small distance from the side of one extremity of 

 the horse-shoe. 



P. S. Last autumn, I commenced a series of observations on the 

 magnetic intensity of the earth at Albany, and intend to begin a new 

 series next month ; the apparatus used was that sent by Capt. Sabine 

 to Prof. Renwick, and was mentioned in the Journal, Vol. xvii, j), 

 145. I have constructed a similar apparatus for myself, and intend 

 to pay considerable attention to the subject. 



^ The armature of 23 lbs. applied when the battciy is iuiineiscd, only lor an incli 

 and an instant, remains, day after day, without falling, although the galvanic coil^ 

 are perfectly dry. — Ed. 



