1831] 



WRITINGS OF JOSEPH HENRY. 



43 



one surface, for an armature or lifter ; the extremities of the 

 legs of the horse-shoe were also truly ground to the surface 

 of the armature : around this horse-shoe 540 feet of copper 

 bell wire wore wound in 9 coils of 60 feet each ; these coils 

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

 each strand of wire, according to the principle before men- 

 tioned, occupied about two inches and was coiled several 

 times backward and forward over itself; the several ends of 

 the wires were left projecting and all numbered, so that the 

 first and the last ei^d of each strand might be readily dis- 

 tinguished. In this manner, we formed an experimental 

 magnet on a large scale, with which several combinations 

 of wire could be made by merely uniting the different pro- 

 jecting ends. Thus, if the second end of the first wire be 

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



[Frame for testing strength of electro-magnet.] 



«, the magnet covered with linen, the ends of the wires projecting so as 

 to be soldered to the galvanic element h . c, a cup with dilute acid on a 

 moveable shelf. di ^ graduated lever, e, a counterpoise. /, a scale for 

 supporting weights ; when a small sliding weight on the lever is not used, a 

 second galvanic element is attached to the apparatus so that the poles of the 

 magnet can be instantly reversed : this is omitted in the figure. 



By inverting the large magnet, it sets in motion a very large revolving 

 cylinder of March and Ampere. 



