Step 1: Gather Your Materials.
For the basic motor, you need a disposable plastic drinking cup, aluminium foil, glue-stick, bamboo or dowel (at least a centimetre or two longer than the cup is tall), wire and a non-conducting base, such as a plastic plate or a wooden board.
Step 2: Making the Rotor.
The spinning part of any motor is called the rotor (because it rotates).
Cut three pieces of foil. The exact dimensions for the basic motor are not important, but they need to be slightly shorter than the cup is tall, and of a width so that there is a gap of about 1cm between the pieces when you stick them to the cup.
Stick the pieces of foil to the cup. Space them evenly around the cup, and make sure there is no point where neighbouring pieces touch each other.
(I say "three pieces of foil", but the exact number seems unimportant - maybe somebody would like to research this point - as long as it is two or more.)
Step 3: Making the Base and Adding the Rotor.
Sharpen the bamboo or dowel to a reasonable point, and then use the blu-tac to stand it in the middle of the base-board. Balance the cup (upside-down) on the bamboo so that it spins freely.
It is possible to sharpen the dowel with a pencil sharpener, or even use a sharp pencil instead of the dowel. Bamboo usually needs a sharp knife.
Step 4: Making the Connections.
Cut two pieces of wire. They need to be long enough to reach your source of static electricity and your earth connection. If you are using balloons to generate the static, give enough clear air between the balloon end of the wire and the motor to prevent draughts from the balloon knocking the cup off its perch.
Position the wires so they stand each side of the cup. They should be about 1cm from the cup (they MUST NOT touch the rotor), pointing at the cup. Fasten the wire in place (how you do this depends on the nature of your base - blobs of blu-tac, drawing-pins, sticky tape, it does not really matter).
Your motor is now finished.
Step 5: Running the Motor.
Connect one wire to connect to a good Earth point - any bare metal part of the home plumbing system is good. If all your water pipes are plastic, poke the wire into th cold-water tap and let it trickle slowly. For your own safety, do not connect the motor to any part of your mains electrical supply.
Now you need a source of static electricity:
If you are lucky enough to own a Van der Graff generator (or you have made your own!), that would be perfect. The earth wire can be connected to the bottom dome of the generator. Otherwise, anything that generates static will do. Inflate a long balloon, rub it on your sweater, and then gently stroke its length down the end of the wire not connected to the Earth. Repeat. Gather several friends, also with balloons, and take turns to add charge to the machine. Lay a sheet of aluminium foil over the screen of a TV or monitor that crackles a lot when you switch it on and off, and use wire to connect the foil to your motor. The more static you can supply, the faster your motor will run. We ran six motors simultaneously from my Van der Graff generator.
Step 6: How It Works.
As you add charge to the unconnected end of the machine, it is transferred to the piece of foil nearest to you, giving it the same charge as the balloons. Like charges repel, so the foil is pushed away from the wire. This turns the rotor.
When the rotor turns, it brings the next piece of foil to collect charge, and also moves the first piece towards the earth connection to discharge. The whole process repeats for as long as you supply charge to the motor. The only real limiters to the speed are balance - wobbly rotors will turn more slowly - and friction. The top speed of your motor will be controlled by friction on the balance point (so make it small and slippery, but don't use grease, try graphite) and by air friction on the cup (so try not to have too many wrinkles or loose edges).