Short answers, pulled from the story.
William Gilbert published De Magnete in 1600 and stated that Earth itself is a magnet. This work established magnetism as a science by mapping the invisible forces surrounding our planet.
The International System unit for B is the tesla equal to newton per meter per ampere. The unit for H measures ampere per meter.
French scholar Petrus Peregrinus de Maricourt mapped magnetic fields on spherical magnets using iron needles in 1269. He identified crossing points as poles and noted magnets always possess north and south ends regardless of size.
Tesla gained patents for his electric motor in May 1888 while Galileo Ferraris published research to the Royal Academy of Sciences in Turin in 1888. Both scientists designed rotating magnetic fields power alternating-current motors independently in the late nineteenth century.
The most recent reversal occurred 780,000 years ago according to geological records. Field strength varies over time while locations shift periodically reversing orientation every few hundred thousand years.