484 MEMORIAL OF JOSEPH HENRY. 



bar and weighed 7 pounds. Nine coils of copper bell-wire, each 60 

 feet in length, were wrapped in sections on the iron. These coils 

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

 strand of wire, according to the principle before mentioned, occupied 

 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 larger 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 through all the series, the Avhole will form a 

 continued coil of one long wire. By soldering different ends the 

 whole may be formed into a double coil of half the length, or into 

 a triple coil of one-third the length, etc. The horse-shoe was sus- 

 pended in a rectangular wooden frame 3 feet 9 inches high and 20 

 inches wide. 



" In order to ascertain the effect of a very small galvanic element 

 on this large quantity of iron, a pair of plates exactly one square 

 inch, was attached to all the wires : the weight lifted was 85 pounds. 

 To find out the greatest supporting power of the magnet, with all 

 of its 9 coils in circuit, a small battery formed of a plate of zinc 

 12 inches long and 6 inches wide, and surrounded by copper, was 

 substituted for the galvanic element used in the former experiments: 

 the weight lifted was 750 pounds." 



The most powerful of Henry's magnets was constructed while he 

 was at Princeton, and is thus described by his successor in the chair 

 of Natural Philosophy, Professor Eichard S. McCulloh: "It is 

 formed of a bar of rounded iron nearly 4 inches in diameter, 

 weighing about 100 pounds, and surrounded with 30 strands of 

 copper bell-wire, each about 40 feet long. With a calorimotor on 

 Dr. Hare's plan, consisting of 22 plates of zinc each 9 inches by 

 12, alternating with plates of copper of the same size, it supports 

 3,500 pounds, or more than a ton and a half. 



"After the connection with the battery is broken, this magnet 

 supports a thousand pounds for several minutes, and from year to 

 year the lifter adheres with a force whidi is overcome only by a 



