John Ambrose Fleming

John Ambrose Fleming (1849–1945) was an English engineer with a genius for making tricky physics stick in your memory — and for building the gadgets that kicked off the electronic age. If you've ever waggled your fingers into a weird claw shape to remember which way a motor pushes, you have met Fleming, even if you didn't know his name.

A mark on the subject

Fleming gave us the famous left-hand rule for the motor effect: point your thumb, first finger and second finger at right angles, and they line up with Force, Field and Current so you can instantly read off which way a current-carrying wire will be pushed in a magnetic field. It's the little hand-trick behind every electric motor. He also invented the vacuum tube diode — the first real electronic component — which made radio and early computers possible.

Fleming worried that students kept mixing up motors and generators, so he invented two hand rules to keep them straight: the left hand for motors (current in, motion out) and the right hand for generators (motion in, current out). It's such a neat memory hack that it has survived, unchanged, in physics classrooms for well over a century. Not every scientist gets a law named after them — Fleming got a permanent piece of your own hands.