242 BULLETIN OF THE 



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

 be developed by a separate wire: In this way the action of each 

 particular coil becomes directed very nearly at right angles to 

 the axis of the bar, and consequently the effect is the greatest 

 possible. This principle is of much greater importance when 

 large bars are used. The advantage of a greater conducting 

 power from using several wires might in a less degree be ob- 

 tained by substituting for them one large wire of equal sectional 

 area ; but in this case the obliquity of the spiral would be much 

 greater, and consequently the magnetic action less."* Moll's 

 single conducting wire of one eighth inch diameter, while there- 

 fore electrically equivalent to about 20 of Henry's conducting 

 wires (of the same length and weight) would be magnetically 

 inferior thereto — for equal iron cores. 



Notwithstanding that Henry's successes were thus both earlier 

 and more brilliant than those of Moll, the two names are usually 

 associated together by European writers in treating of the deve- 

 lopment of the magnet, f 



Among the subsequent experiments on which Henry was en- 

 gaged at the time of receiving the Edinburgh Journal of Science 

 containing Moll's paper, was a series on a much larger magnet, 

 consisting of a bar of soft iron two inches square (with the 

 angles rounded) and twenty inches long, bent into a horse-shoe 

 about nine inches high, and weighing 21 pounds. Its armature — 

 a piece from the same bar ground to fit truly the ends of the 

 horse-shoe, weighed 7 pounds. Nine coils of copper bell-wire 

 each 60 feet in length (making 540 feet in all) were separately 

 wound on different portions of the horse-shoe. " These coils 

 were not continued around the whole length of the bar, but each 

 strand of wire according to the principle befoi'e mentioned, occu- 

 pied about two inches, and was coiled several times backward 

 and forward over itself: the several ends of the wire were left 

 projecting and all numbered, so that the first and last end of 

 each strand might be readily distinguished. In this manner was 

 formed an experimental magnet on a large scale, with which 

 several combinations of wire could be made by merely uniting 

 the different projecting ends. Thus if the second end of the first 

 wire be soldered to the first end of the second wire, and so on 



* Sill. Am. Jour. Sci. January, 1831, vol. xix. p. 402. The three 

 names — Arago, Sturgeon, and Henry, may well typify the infancy, the 

 youth, and the mature manhood of the electro-magnet. 



f Faraday in subsequently investigating the conditions of galvanic 

 induction, referred with approbation to the magnets of Moll and lleniy 

 as best calculated to produce the effects sought. In constructing liis 

 duplex helices for observing the direction of the induced current, he 

 however adopted Henry's method by winding 12 coils of copper wire each 

 27 feet long — one upon the other. {Phil. Trans. Roy. Soc. Nov. 24,1831, 

 vol. cxxii. (for 1832,) pp. 12(), and 138. Experimental Researches, etc, 

 vol, i. art. (J, p. 2 ; and art. 57, p. 15 ) 



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