1831] WRITINGS OF JOSEPH HENRY. 47 



Exp. 23. A piece of a gun-barrel, little less than inch in 

 diameter and about 8 inches long, and from y^^ to ^ of an 

 inch thick, weighing 8 oz. 3f dr. (with the wires and battery 

 as before,) lifted 40 lbs. 



From the last experiment, it appears that a given quantity 

 of iron in the form of a hollow cylinder, is capable of receiv- 

 ing more magnetism than that of a solid cylinder of less 

 diameter; but it is evident from Exp. 21, that a solid bar of 

 the same diameter as the gun-barrel, and of greater weight, 

 would have lifted more: perhaps the gun-barrel was not 

 sufficiently thick for the full development of magnetism, 

 which, according to Barlow's experiments, resides near the 

 surface.* 



A series of experiments f was separately instituted by 

 Dr. Ten Eyck in order to determine the maximum devel- 

 opment of magnetism in a small quantity of soft iron : 

 from these the following interesting results were obtained. 



Experiment 1. A horse-shoe of round iron -j^^ of an inch 

 in diameter, 4 inches long, weighing 2314 grains and wound 

 with 23 ft. copper-wire, diameter Yrffo" ^^ ^^ inch, with a 

 pair of one inch plates, lifted 19 lbs. 5 oz. 6 dwt. 16 grs.; 

 with a pair of 4 inch plates, lifted 25 lbs. 6 oz. 5 dwt.; with 

 the cylindrical element used in Exps. 8, 9 and 10 of former 

 series, it lifted 42 lbs. 6 oz. 8 dwt. 8 grs., or 105 times its own 

 weight. 



Exp. 2. A horse-shoe of round iron \ inch in diameter, 3| 

 inches in length weighing 310 grains, and wound with 15 

 ft. copper wire, diameter yMt inch, with a pair of one inch 

 plates, lifted 3 lbs. 11 oz. 7 dwt. 22 grs.; with 4 inch, plates 

 it lifted 5 lbs. 5 oz. 12 dwt. 12 grs.; with the cylindrical ele- 

 ment 8 lbs. 2 oz. 8 dwt. 18 grs., or 152 times its own weight. 



Exp. 3. A horse-shoe formed of a flat bar 'l-^^ inches long 

 ^V in. broad and yf ^ in. thick, weighing 84 grains, and wound 

 with 16 feet of brass wire, y|o of an inch in diameter, with 

 a pair of one inch plates, lifted 5 lbs. 2 oz. 3 dwt. 8 grs. ; 



*See Barlow's Essay on Magnetic Attractions, page 50. 

 f Troy weight is used in these experiments. 



