When a current-carrying conductor is placed in an external magnetic field, the conductor experiences a force perpendicular to both the field and to the direction of the current flow.
It was invented by John Ambrose Fleming.
Illustration using the left hand
A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, forefinger, and middle finger. If we arrange the thumb, the center finger, and the forefinger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the center finger gives the direction of current and the forefinger points in the direction of a magnetic field.