118 Trans. Acad. Sci. of St. Louis. 



directive action of currents upon magnetic needles, and upon 

 other currents. Out of these studies grew the needle tele- 

 graph. 



Sturgeon was the first to intensify the magnetic field of a 

 current, by winding it in a coil around the legs of an iron bar 

 bedt into the horse-shoe form. This was an electro-magnet. 

 Joseph Henry produced an electro-magnet of enormous lifting 

 force, and used small electro-magnets to send signals to a 

 distant point. He connected his battery cells and designed 

 the magnet-windings so as to make the signals effective over 

 a long circuit. The telegraph line which he established at 

 Albany made signals by means of the sound of the attracted 

 armature striking a resonant stop. Some years later Morse 

 added a system of recording apparatus, and built a line for 

 commercial purposes. In practice the signals by means of 

 sound were found more convenient than those which Morse 

 tried to introduce. 



During the greater part of the century, the source of elec- 

 tricity was the battery, of Volta and Galvani. But as early 

 as 1831, Faraday had made the grand discovery, which was 

 to work a revolution. On September 22 of that year he wrote 

 in his laboratory note-book as follows : — 



" I have had an iron ring made, (soft iron) iron round and 

 f of an inch thick, and ring six inches in diameter. Wound 

 many coils of copper round, one half of the coil being sep- 

 arated by twine and calico. There were three lengths of wire, 

 each about 24 feet long, and they could be connected as one 

 length or as separate lengths. By trial of a trough, each was 

 insulated from the other. We will call this side of the ring 

 A. On the other side, but separated by an interval, was 

 wound wire in two pieces, together amounting to about 60 feet 

 in length, the direction being as with the other coils. This 

 side call B. 



" Charged a battery of ten plates, four inches square, made 

 the coil on B side one coil and connected its extremities by a 

 copper wire passing to a distance and just over a magnetic 

 needle (three feet from wire ring). Then connected the ends 

 of one of the pieces on A side with the battery ; immediately 

 a sensible effect on needle. It oscillated and settled at last in 



