1831] WRITINGS OF JOSEPH HENRY. 39 



was filled with dilute acid, the bar became a portable electro- 

 magnet. These articles were exhibited to tlie Institute in 

 March, 1829. 



The idea afterwards occurred to me, that a sufficient quan- 

 tity of galvanism was furnished by the two small plates, to 

 develope, by means of the coil, a much greater magnetic 

 power in a larger piece of iron. To test this, a cylindrical 

 bar of iron, | an inch in diameter, and about 10 inches 

 long, was bent into the form of a horse-shoe, and wound 

 with 30 feet of wire; with a pair of plates containing only 

 2J square inches of zinc, it lifted 14 lbs. avoirdupois. At 

 the same time, a very material improvement in the forma- 

 tion of the coil suggested itself to me, on reading a more 

 detailed account of Prof. Schweigger's galvanometer, and 

 which was also tested with complete success upon the same 

 horse-shoe; it consisted in using several strands of wire, 

 each covered with silk, instead of one : — agreeably to this 

 construction, a second wire, of the same length as the first, 

 was wound over it, and the ends soldered to the zinc and 

 copper in such a manner that the galvanic current might 

 circulate in the same direction in both, or in other words, 

 that the two wires might act as one; the effect by this addi- 

 tion was doubled, as the horse-shoe, with the same plates 

 before used, now supported 28 lbs. 



With a pair of plates 4 inches by 6 inches, it lifted 39 lbs., 

 or more than 50 times its own weight. 



These experiments conclusively proved that a great de- 

 velopement of magnetism could be effected by a very small 

 galvanic element, and also that the power of the coil was 

 materially increased by multiplying the number of wires, 

 without increasing the length of each. 



The multiplication of the wires, increases the power in 

 two ways; first, by conducting a greater quantity of gal- 

 vanism, and secondly, by giving it a more proper direction, 

 for since the action of a galvanic current is directly at right 

 angles to the axis of a magnetic needle, by using several 

 shorter wires, we can wind one on each inch of the length 

 of the bar to be magnetized, so that the magnetism of each 



