Who first measured magnetic force?
History of geomagnetism
Magnetism has been known since prehistory, but knowledge of the Earth's field developed slowly. The horizontal direction of the Earth's field was first measured in the fourth century BC but the vertical direction was not measured until 1544 AD and the intensity was first measured in 1791. At first, compasses were thought to point towards locations in the heavens, then towards magnetic mountains. A modern experimental approach to understanding the Earth's field began with "de Magnete", a book published by William Gilbert in 1600. His experiments with a magnetic model of the Earth convinced him that the Earth itself is a large magnet.
AC motor
Because of AC's advantages in long-distance high voltage transmission, there were many inventors in the United States and Europe during the late 19th century trying to develop workable AC motors. The first person to conceive of a rotating magnetic field was Walter Baily, who gave a workable demonstration of his battery-operated polyphase motor aided by a commutator on June 28, 1879, to the Physical Society of London. Describing an apparatus nearly identical to Baily's, French electrical engineer Marcel Deprez published a paper in 1880 that identified the rotating magnetic field principle and that of a two-phase AC system of currents to produce it. Never practically demonstrated, the design was flawed, as one of the two currents was “furnished by the machine itself.” In 1886, English engineer Elihu Thomson built an AC motor by expanding upon the induction-repulsion principle and his wattmeter. In 1887, American inventor Charles Schenk Bradley was the first to patent a two-phase AC power transmission with four wires.