A rotary high voltage motor
Click on the photo to see an animated picture
At one company I once worked for, we had a contest for
"The most creative use of office supplies". This toy
would clearly be a contender. It is based on a wonderful design
by Bill Beaty, but this version uses only things found around
the typical office coffee room.
Using the safe high voltage power we get by placing a sheet of
aluminum foil on the face of a television or computer CRT
screen, it spins a styrofoam cup around at a respectable speed.
To build the toy, you need:
- Two empty soda cans.
- A styrofoam cup (a paper cup will also work).
- A ball-point pen (the simple non-clicking type).
- A couple square feet of aluminum foil.
- Two paper clips
- A hot glue gun (or regular glue if you don't mind waiting).
- Cellophane tape.
- Two wires (alligator test leads work great).
We start by spreading glue over the ouside of the styrofoam cup.
Put just a thin layer on, so it dries quickly. Before it dries,
cover the cup with aluminum foil. Press the foil flat against
the cup, so any wrinkles are pressed down.
With a sharp knife, neatly cut a half inch strip out of the foil
on both sides, so you have two patches of foil, one on each side
of the cup, that do not touch one another.
The cup is going to be spinning upside down on the point of a
ball-point pen. To keep the cup centered on the pen-point,
and to provide a low friction bearing, we need to glue something
hard to the center of the bottom of the cup, something that
has a little dimple in it to sit on the pen-point.
I chose to sacrifice the end of another ball-point pen.
The photo shows the end of the pen, cut off with a sharp knife.
The side of the cut end that is facing down has a little dimple
that is perfect as a place to accept the point of the other
The end of the pen is glued in the exact center of the bottom
of the cup, as shown below. Note the little dimple.
Next we make the stand for the motor. Start with a paper plate, and
glue the bottom of a ball point pen to the exact center of the plate,
so the point stands straight up.
Glue the two soda cans upside-down onto the plate, leaving enough
room between them for the strofoam cup to rotate easily without
touching either can. There should ba about a half of an inch gap
between the cup and either can.
Straighten two of the bends of a paper clip (leaving one end bent
as in the photo below) and tape them to the cans as shown. Bend
the wires into an S shape, leaving enough room to place the cup
on top of the pen.
Now put the cup upside-down onto the pen-point. Make sure the
dimple fits onto the pen-point. The wires should be about a half
inch away from the cup, with the point being closest to the cup.
Nothing should be touching the cup except the point of the pen.
Now connect a wire from the can on the right to a large sheet of aluminum
foil pressed against the screen of a TV (or a computer with a CRT screen).
Connect another wire to the left can, and connect the free end to a
good ground connection, such as a cold water pipe, or the metal frame
of a computer. In a pinch, you can just hold onto the free end,
since your body is a good enough ground for this little motor.
When you turn on the television, the foil will pick up a high voltage,
and the little motor will start spinning. As it slows down, turn the
television off, and the motor will get another kick, and spin faster.
You can keep this up as long as you feel like turning the TV on and off.
How does it do that?
The can on the right is charged with high voltage from the
face of the TV. This means that the electrons are pushed
onto the can with a lot of electrical "pressure". The electrons
all have the same negative charge, and so they repel one another.
At the point of the wire the electrone are most crowded, and so
there is the highest pressure. It is so high that the electrons
can get pushed right off the wire, landing on the air molecules
near the wire. These air molecules are now also negative, so they
are repelled by the extra electrons in the wire. They move away
from the wire, and hit the aluminum foil, where the electrons
leave the air molecules and collect on the foil.
The electrons on the foil are repelled by the can on the right,
which still has an excess of electrons. The electrons in the
can on the left are repelled by the electrons in the foil, so
they move away from it, leaving behind the positively charged
nucleus, which attracts the electrons in the foil.
With the right can repelling, and the left can attracting, the
foil is pulled to the left, and the cup rotates.
Half a revolution later, the charged foil is next to the wire
that is attached to the ground. The electrons from the foil
charge the air around the wire, and the electrons can move from
the foil through the air and the wire, to the ground. This leaves
the foil uncharged, so it is not repelled by the can on the right
as it moves by.
At this point we are back to where we started, and the process
starts over again.
Next: A simple Van de Graaff high voltage generator
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Simon Quellen Field