THE CANTOR LECTURE, i 890. 
49 
when this cup, which served the double purpose of a stand and the 
galvanic element, was filled with dilute acid, the bar became a portable 
electro-magnet. These articles were exhibited to the Institute in 
March, 1829. The idea afterwards occ.ured to me that a sufficient 
quantity of galvanism was furnished by the two small plates to develop, 
by means of the coil, a much greater magnetic power in a larger piece 
of iron. To test this, a cylindrical bar of iron, \ inch in diameter, and 
about 10 inches long, was bent into the shape of a horse-shoe, and 
wound with 30 feet of wire; with a pair of plates containing only 2h 
square inches of zinc, it lifted 15 lbs. avoirdupois. At the same time, 
a very material improvement in the formation 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 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 
addition 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 fifty times its own weight. These 
experiments conclusively proved that a great development of mag¬ 
netism 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 number of each.” 
Not content with these results, Professor Henry pushed forward on 
the line he had thus struck out. He was keenly desirous to ascertain 
how large a magnetic force he could produce when using only currents 
of such a degree of smallness as could be transmitted through the com¬ 
paratively thin copper wires, such as bell-hangers use. During the 
year 1830 he made great progress in this direction. 
In Silliman’s Journal (April, 1831), Professor Henry gave “an 
account of a large electro-magnet, made for the laboratory of Yale 
College.” The core of the armature weighed 59J lbs., it was forged 
under Henry's own direction, and wound by Dr. Ten Eyck. This 
magnet, wound with 26 strands of copper bell-wire of total length of 
728 feet, and excited by two cells which exposed nearly 4-J square feet 
of surface, readily supported on its armature, which weighed 23 lbs., a 
load of 2,063 lbs. 
Writing in 1867 of his earlier experiments, Henry speaks thus of 
his ideas respecting the use of additional coils on the magnet and the 
increase of battery power :— 
“ To test these principles on a larger scale, an experimental magnet 
was constructed. In this a number of compound helices w r as placed on 
the same bar, their ends left projecting, and so numbered that they 
could all be united into one long helix, or variously combined in sets 
of lesser length. 
From a series of experiments with this and other magnets, it was 
proved that in order to produce the greatest amount of magnetism from 
