126 
in 18rg,* had observed the tendency of a magnetic needle to place 
itself perpendicular to a wire conveying an electrical current. 
Ampérey had studied the mutual action of currents upon each other, 
and had thus created the science of electrodynamics. Schweigger 
had multiplied the number of convolutions of the wire about the 
needle, increasing proportionately in this way the effect.t Arago 
had succeeded in producing magnetism from an electrical current 
by winding the wire carrying this current in a loose helix and 
placing pieces of iron wire in the axis of this helix; thus creating 
the ‘‘electromagnet.’’§ Sturgeon had further developed this idea 
by coating the iron bar, which was bent into ahorseshoe form, with 
a non-conducting substance, and winding the wire directly on the 
bar, thus increasing the closeness of the contact.|| Henry’s first 
paper in electric science was a communication made to the Albany 
Institute, October 10, 1827, ‘‘On Some Modifications of the Elec- 
tromagnetic Apparatus.’’4] In this paper he suggested several im- 
provements in the construction of the electromagnet, which greatly 
increased its efficiency. In the first place he adopted the multiple 
arrangement of turns proposed by Schweigger in his galvanometer ; 
and in the second, instead of insulating the bar to be magnetized, 
he insulated the conducting wire itself, covering the whole surface 
of the iron with a series of coils in close contact. Sturgeon’s 
electromagnet of 1825 consisted of a stout iron wire bent into a U 
form, having a copper wire wound loosely round it, forming 
eighteen turns. Henry’s electromagnet of 1829 was made of 
a piece of round iron about one-quarter of an inch in diameter, 
bent into the form of a horseshoe, and tightly wound with 
thirty-five feet of wire covered with silk, forming about four 
hundred turns. Later in the same year, Henry still further 
increased the power of his electromagnet, by winding the wire 
upon the iron core, not in a single strand, but in several; the 
current flowing simultaneously through the different strands. 
Using a U-shaped bar of iron, half an inch in diameter and 
about ten inches long, wound with thirty feet of tolerably fine 
copper wire, he observed that with a cell containing two and 
* Schweigger, J., XXix, 273, 1820; Gilb. Ann., 1xvi, 295, 1820. 
t Ann. Chim. Phys., xv, 59, 170, 1820; xviii, 88, 813, 1821; xxvi, 390, 1824. 
{ Allgem. Literaturzeitung, No. 296, Nov., 1820; Schweigg., J., xxxi, 12, 1826. 
2 Ann. Chim. Phys., XV, 93 (1820). 
|| Trans. Soc. Encour. Arts, xliii, 38 (1825). 
{ Trans, Albany Institute, Vol. i, pp. 22, 23 (1827). 
