PHILOSOPHICAL SOCIETY OF WASHINGTON. 241 



strands of fine silk-covered wire, each about 30 feet long ; which 

 arrangement when placed in the circuit of a single galvanic pair 

 whose zinc surface was 6 inches by 4 inches (one-sixth of a 

 square foot) sustained by its armature " 89 pounds, or more than 

 fifty times its own weight;" Moll's highest result (of which he 

 justly felt proud) being only fifteen times the weight of the 

 magnet, with 11 square feet of zinc surface. While Henry's 

 magnet had the practical advantage of being about only one-half 

 the size of Moll's — in each dimension, (and therefore about only 

 one-eighth its weight without wrappings,) yet it supported more 

 than half his load : (39 pounds to 15 pounds.) Moll had em- 

 l)loyed a single copper wire one-eighth inch thick and about 22 

 feet long : Henry, several strands each about one thirty-sixth 

 of an inch thick, and 30 feet long; — the former making 83 turns 

 around the iron core, — the latter, several hundred turns. But 

 the most surprising contrast resulting from these differences was 

 the enormous difference of battery-power applied ; Moll pushing 

 his up to 17 square feet, — Henry reducing his to one-sixth of one 

 square foot. With a galvanic element reduced to two and a 

 half square inches, his magnet sustained 28 pounds ; or more 

 than double the relative duty of Moll's at its highest power. 

 The philosopher of Utrecht, though he evidently realized with 

 him of Albany, the importance of close winding, employed but a 

 single layer of coil. The latter, by means of well-considered trials 

 had ascertained the great increase of magnetic force resulting 

 from a succession of coils. 



To Henry therefore belongs the exclusive credit of having first 

 constructed the magnetic " spool" or " bobbin" : that form of coil 

 since universally employed for every application of electro-mag- 

 netism, of induction, or of magneto-electrics. This was his first 

 great contribution to the science and to the art of galvanic mag- 

 netization. It may be very confidently affirmed that prior to 

 1829, no one on either hemisphere had ever thought of winding 

 the legs of an electro-magnet on the principle of the "bobbin"; 

 and that not till after the publication of Henry's method in Janu- 

 ary of 1831, was it ever employed by any European physicist. 



" These experiments conclusively proved that a great develop- 

 ment 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 galvanism, and secondly, by giving it a more proper direc- 

 tion ; 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 

 VOL. II. — 16 15 



