by such currents.
Curiously, even in 1802 the fact that a wire conveying a current would deflect a compass needle was observed by Romagnosi, of Trente, but it was afterwards forgotten, and not until 1819 was any real advance made.
It was then that Oersted, of Copenhagen, showed that a magnet tends to set itself at right angles to the wire conveying current and that the direction of turning depends on the direction of the current.
The study of the magnetic effects of electric currents by Arago, Ampere, and the production of the electro-magnet by Sturgeon, together with the very valuable work of Henry and others, made possible the completion of the electric telegraph.
This was done by Morse and Vail in America, and almost simultaneously by workers abroad, but, before Morse had entered the field, Prof. Joseph Henry had exemplified by experiments the working of electric signalling by electromagnets over a short line.
It was Henry, in fact, who first made a practically useful electr