A current sent through a wire in the second story of the building induced currents through a similar wire in the cellar two floors below.

In this discovery Henry anticipated Faraday though his results as to mutual induction were not published until he had heard rumors of Faraday's discovery, which he thought to be something different.
The attempt to send signals by electricity had been made many times before Henry became interested in the problem.

On the invention of Sturgeon's magnet there had been hopes in England of a successful solution, but in the experiments that followed the current became so weak after a few hundred feet that the idea was pronounced impracticable.

Henry strung a mile of fine wire in the Academy, placed an "intensity" battery at one end, and made the armature strike a bell at the other.

Thus he discovered the essential principle of the electric telegraph.