Three directions,
one right angle each
The hand rules are a memory aid for a cross product. Here it is as geometry instead: set the field and the current, and the third direction is forced. The two hands are the same rule read in opposite causal order — force from current, or current from motion.
whatA wire segment in a magnetic field, with the three mutually perpendicular directions drawn on the same axes the hand rules describe.
lawF = IL × B. Left hand for the motor effect: field, current, force. Right hand for the generator: field, motion, induced current.
tryDrag the current dial around, then reverse the field.
noticeReversing either input flips the result; reversing both leaves it unchanged. The cross product is zero when current and field are parallel — no angle, no force.
in lifeWired the wrong way round, a motor runs backwards — this is the check that catches it.
found byJohn Ambrose Fleming devised the hand rules in the 1890s as a teaching aid for his students.