284 JOSEPH HENRY, LL.D. 



Henry improved upon this in 1828, but in 

 March of 1829 he exhibited before the Institute a 

 somewhat larger magnet. " A round piece of iron 

 about one-quarter of an inch in diarneter was bent 

 into the usual form of a horse-shoe, and, instead of 

 loosely coiling around it a few feet of wire as is 

 usually described, it was tightly wound with thirty- 

 five feet of wire covered with silk, so as to form 

 about four hundred turns ; a pair of small galvanic 

 plates, which could be dipped into a tumbler of 

 diluted acid, was soldered to the ends of the wire, 

 and the whole mounted on a stand. With these 

 small plates, the horse-shoe became much more 

 powerfully magnetic than another of the same size 

 and wound in the usual manner, by the application 

 of a battery composed of twenty-eight plates of 

 copper and zinc each eight inches square." 



"To Henry, therefore," says Mr. Taylor, "be- 

 longs the exclusive credit of having first con- 

 structed the magnetic ' spool' or 'bobbin/ that 

 form of coil since universally employed for every 

 application of electro-magnetism, of induction, or 

 of magneto-electrics. This was his first great con- 

 tribution to the science and to the art of galvanic 

 magnetization. . . . 



"But, in addition to this large gift to science, 

 Henry has the preeminent claim to popular grati- 

 tude of having first practically worked out the 

 differing functions of two entirely different kinds 

 of electro-magnet ; the one surrounded with numer- 

 ous coils of no great length, designated by him 



