1857] WRITINGS OF JOSEPH HENRY. 427 



Third, that in which electro-magnetism is the motive 

 power to produce motion at a distance: and again, of the 

 latter there are two kinds of telegraphs, those in which the 

 intelligence is indicated by the motion of a magnetic needle 

 and those in which sounds and permanent signs are made 

 by the attraction of an electro-magnet. The latter is the 

 class to which Mr. Morse's invention belongs. The follow- 

 ing is a brief exposition of the several steps which led to 

 this form of the telegraph : 



The first essential fact (as I stated in my testimony) that 

 rendered the electro-magnetic telegraph possible was discov- 

 ered by Oersted, in the winter of 1819-20. It is illustrated 

 by figure 1, in which the magnetic needle is deflected by the 



Fig. 1. 



action of a current of galvanism transmitted through the 

 wire A B. (See Annals of Philosophy, Oct., 1820, vol. xvi, 

 page 274.) 



The second fact of importance, discovered in 1820 by 

 Arago and Davy, is illustrated in figure 2. It consists in 



Fig. 2. 



this, that while a current of galvanism is passing through a 

 copper wire A B, it is quasi magnetic, that is, it attracts iron 

 filings in a cylindrical sheath around it, and not those of 

 copper or brass, developing magnetism in soft iron. (See 

 Annates de Chimie et de Physique, 1820, vol. xv, page 94.) 



The next important discovery, also made in 1820, by 

 Ampere, was that two wires through which galvanic currents 

 are passing in the same direction attract — and in the oppo- 

 site direction repel each other. On this fact Ampere founded 



