... Files in red?history) . ... Magnetic Field . ... It was believed that the inside of the Earth was magnetized in the same fashion, and scientists were greatly puzzled when they found that the direction of the compass needle at any place slowly shifted, decade by decade, suggesting a slow variation of the Earth's magnetic field. ... Andr -Marie Amp re in France felt that if a current in a wire exerted a magnetic force on a compass needle, two such wires also should interact magnetically. ...