John Ambrose Fleming introduced two rules to determine the direction of motion in motors or the direction of induced current in generators. The rules are called as Fleming’s left-hand rule for motors and Fleming’s right-hand rule for generators.
Fleming’s Left-Hand Rule
Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force which is perpendicular to both the magnetic field and the direction of the current. According to Fleming’s left-hand rule, if the thumb, forefinger and middle finger of the left hand are stretched to be perpendicular to each other as shown in the illustration at left, and if the forefinger represents the direction of magnetic field, the middle finger represents the direction of current, then the thumb represents the direction of force. Fleming’s left-hand rule is applicable for motors.
Fleming’s Right-Hand Rule
Fleming’s right-hand rule is applicable for electrical generators. As per Faraday’s law of electromagnetic induction, whenever a conductor is forcefully moved in an electromagnetic field, an emf gets induced across the conductor. If the conductor is provided a closed path, then the induced emf causes a current to flow. According to Fleming’s right-hand rule, the thumb, forefinger and middle finger of the right hand are stretched to be perpendicular to each other as shown in the illustration at right, and if the thumb represents the direction of the movement of conductor, fore-finger represents direction of the magnetic field, then the middle finger represents direction of the induced current.