An Etymological Dictionary of Astronomy and Astrophysics

Two rules used to assist in remembering the relative directions of the magnetic field, current, and motion in electrical machines, using one’s fingers. The right hand refers to generators, the left hand to motors. The three directions are represented by the thumb (for force or motion), forefinger (for field), and second finger (for current), all held at right angles to each other.

See also: Devised by the British physicist and electrical engineer John Ambrose Fleming (1849-1945).