1831] WRITINGS OF JOSEPH HENRY. 53 



pieces of round iron IJ inches in diameter and 12 inches 

 long, were interposed 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 power- 

 fully magnetic as to support 155 lbs. 



To exhibit the effects produced by instantaneously revers- 

 ing the poles, the armature was loaded with 56 lbs., which 

 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 was then dipped into the acid and immediately withdrawn, 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 mag- 

 net; the other battery was then suddenly immersed, 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 large 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 in- 

 tend 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, p. 145. I have con- 

 structed a similar apparatus for myself, and intend to pay 

 considerable attention to the subject. 



