Who discovered this and its use?

WILLIAM GILBERT?

The first attempt to separate fact from superstition came in 1269, when a soldier named Peter Peregrinus wrote a letter describing everything that was known, at that time, about magnetite. It is said that he did this while standing guard outside the walls of Lucera which was under siege. While people were starving to death inside the walls, Peter Peregrinus was outside writing one of the first 'scientific' reports and one that was to have a vast impact on the world. It wasn't until the experiments of William Gilbert in 1600 that significant progress was made in the understanding of magnetism and it was another century or so before other scientists began, by experimentation, to understand the phenomenon.

Thanks so much Mozart! Great answer..