36 PROFESSOR HENRY AND THE 



graph, which in a practical and operative form 

 was exhibited to the Emperor Alexander in 1824, 

 and came to be well known to scientific persons 

 at that time.* 



In 1833, Gauss and Weber set up a single circuit 

 galvanometer telegraph on this plan at Gottengen, 

 leading the wire over the house-tops, on insula- 

 tors, as we do now; and by the deflections of the 

 needle to the right and left made up the alphabet, 

 as it had been done before when using other means 

 for moving the vanes, f 



Their apparatus, however, is perfectly silent. 

 The needle is suspended by a thread when the noise- 

 less current sways it to and fro with but feeble 

 force; and it is incapable of calling the attention of 

 the operator to receive its message. These were 

 serious difficulties, to be overcome by other princi- 

 ples, and other inventions, which would supersede 

 this one. 



Following Oersted, Arago, in France, in 1820, 

 made the next capital discovery. It was but 

 a little thing he saw simply that a sewing needle, 

 surrounded by a coil of wire, through which a vol- 

 taic current passed, had become magnetic; but that 

 little thing has grown to be mighty. This obser- 

 vation was the complete discovery of electro-mag- 

 netism, which had been dimly seen in Oersted's gal- 



* See Appendix, Note L. 

 t See Appendix, Note M. 



