Build a Feynman Reverse
Sprinkler
Allan Rydberg, Research Engineer, Brown University
For those that haven't heard of Richard
Feynman, I suggest you check out the local library or
the web for some of his lectures and books. They range
from a classic theoretical text ("The Feynman Lectures
in Physics") to humorous accounts of his life.
One of the puzzles he posed was what
would happen if you took a rotating garden sprinkler
and placed it in the bottom of a swimming pool and attached
the hose to the intake of a pump so the water could
be pumped out from the sprinkler. The question is would
the sprinkler turn? If so, would it turn in the opposite
direction from normal?
If you Google on "Feynman reverse
sprinkler" you will find some of the debate on
this issue.
This article is a description of how
to build your own Feynman Reverse Sprinkler. The parts
are easily available. Some soldering is required. The
design incorporates a magnetic bearing to reduce the
friction of the shaft.
Materials
1 3-liter soda bottle.
1 2-hole cork to fit bottle.
1 piece of brass tubing 30.5 cm (12
inches) long. The size is chosen to fit into the holes
in the cork.
1 piece brass tubing 30.5 cm (12 inches)
long to nest inside the piece above.
(The above 2 items can be found in
most hobby shops.)
4 #10 steel fender washers.
1 large pipe plug 1/8-inch (or metric
equivalent) pipe thread This plug has a 7/16-inch hex
head.
2 smaller pipe plugs also to fit 1/8-inch
(or metric equivalent) pipe thread. Note that these
have a four sided top and are smaller than the one above.
1 brass tee 1/8-inch (or metric equivalent)
pipe thread.
2 close nipples 1/8-inch (or metric
equivalent) pipe thread.
2 elbows 1/8-inch (or metric equivalent)
pipe thread.
one brass washer to fit over the larger
brass tubing.
(The above parts are available in hardware
stores.)
12 cubical magnets 0.5 cm (3/16-inch)
on each side. (Available from www.amazingmagnets.com.)
One collar to fit on the brass tube.
(This should be available at some hobby shops. Mine
was from Stock Drive Products (Number S3402Y-C229) and
cost about $12 US. I suggest getting it as a last resort.
Also I had to shim it with a short length of the larger
brass tube.)
Assembly
Refer to Fig. 1 for the following steps.
1) Cut two 5-cm (2-inch) lengths of
the larger brass tubing. Cut the ends at a 45-degree
angle and solder together. Then insert in the cork.
2) Cut one 5 cm (2-inch) lengths of
the larger brass tubing. Solder the brass washer over
the tubing about 6.4 mm (1/4 inch) from the end of the
tubing. Insert the tubing into the cork with the washer
on the outside of the bottle.
3) Drill out two of the steel washers
to fit over the larger tubing With long nose pliers
place six of the magnets on the washer and place the
second washer on top. The magnets will hold all the
parts together. Then place the magnet assembly over
the brass tube.
4) Repeat step three with the 2 undrilled
steel washers. These two disks form a magnetic thrust
bearing. The washers should fit over the smaller tubing.
5) Drill a hole in the large pipe plug
to allow the smaller tubing to fit inside and solder
the tubing to the pipe plug. It is important that the
plug is not tilted on the brass tube. The hole should
be a close fit and a drill press or lathe may be required
to drill the hole straight.
6) Insert the small tubing into the
larger tubing and place the magnet assembly so the poles
repel. then place a collar on top of the magnet.
This is all shown in Fig. 2.
7) Thread the two close nipples into
the opposite ends of the tee and add the elbows to the
ends of the nipples. Be sure the elbows face in opposite
directions.
8) Thread the two pipe plugs with the
square tops in to the elbows. The plugs must then be
cut shorter so the assembly will fit into the mouth
of the of the 3-liter bottle. They can be cut with a
hacksaw. It is easier if they are first tightened into
the elbows and then the assembly can be held in a vise.
After cutting the square tops test it to see that it
fits into the bottle. If it does then file it flat and
drill a hole in the center. I first used a #33 drill.
If necessary mouth of the bottle can be distorted by
squeezing it with a vise but it is better if the sprinkler
goes in and out easily.
9) Push the spinner into the bottle.
Hold the bottle upside down and juggle it until the
sprinkler is centered and you can stick the brass tube
into the bottle and thread it into the sprinkler. This
is not as difficult as it sounds.
See Fig. 3 for the finished bottle.
Testing the Fenyman Sprinkler
Carefully inspect your apparatus to
be sure it is properly assembled and then test it.
Does your sprinkler rotate? If so,
in what direction?
Readers who assemble and test the
Fenyman Reverse Sprinkler system are encouraged to send
their results here
for possible publication in The Citizen Scientist.
Editor. 
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