- Building simple heat engines.
- Hero's steam engine.
- A simple steam engine powered boat.
- A simple rotary steam engine.
- An engine driven by the heat of your hand.
- A bi-metal strip heat engine.
A simple solar powered heat engine
- A simple rocket engine you can build in your kitchen.
- Building a Film Can Cannon.
- A metal that melts in hot water.
Liquid metal at room temperature.
Building the Film Can Cannon
This toy was an instant favorite from the moment its first
loud Bang! and flash of orange flame launched the
little black film can up to bounce off our 26 foot ceiling.
It has several names: the Piezo-Popper, the Binaca Bomb,
the Photo-Flash -- you will probably come up with more.
The toy is very easy to make, going together in about
15 minutes, at a cost of two or three dollars if all
the parts are purchased new, or free if you don't throw
away certain common household items.
The fuel for the cannon can be found around the house.
We have run ours on perfume, hair spray, and (our
favorite) Wintergreen Binaca mouth freshener.
The cannon is very simple. A pair of wires are pushed
through a slit made in the top of a plastic 35 millimeter
film can. The other end of the wires are soldered to
the igniter element from an electronic cigarette or
fireplace lighter. I chose to mount these elements on
a block of wood, but this is optional if you're in a
To fire the cannon, you spray the fuel (one squirt of perfume
or Binaca, or a very short squirt of hair spray) into the
plastic film can, push the can down on the lid, and press the
With a loud Bang! and a flash of orange flame, the little
can goes sailing into the air. With some practice in getting
just the right amount of fuel in the can, it will go as high as
30 feet straight up. If too much or too little fuel is used, it
will either not ignite at all, or it will not go very high.
The finished cannon is shown above, next to the first fuel we tried,
a bottle of perfume purloined from an unsuspecting donor.
Here it is with the can removed from its launch pad.
You can see the two wires coming through the slit in
The block of wood has three holes drilled in it. Two go all
the way through, so the wire can be threaded through them.
The other hole does not go all the way through, and it used
to hold the igniter.
Here is the bottom view, showing the wire threaded through
the holes. You can also see the stick-on rubber feet that
keep the wire from making the block wobble. You can get
these feet at Radio Shack, or at a cabinetry supply shop
or hardware store.
Here is a closeup of the spark gap formed by the stripped
ends of the two wires. There is nothing critical about this
arrangement -- as long as the wire ends are bare and close
enough, a spark can jump across the gap when the igniter is
Here is a closeup of the igniter, showing the wires soldered to
its contacts. There are many types of igniters in the different
brands of electronic lighters.
Above is the igniter from a small
Scripto Electronic lighter.
It is smaller, and not as sturdy as the ones from the fireplace
lighters, but the small lighters are usually less expensive.
Below is a small lighter disassembled to show the igniter.
The lighters come apart easily without tools.
Below is the igniter from a large fireplace lighter (I have
found some non-brand-name lighters at a hardware store for
$1.99, but the brand name versions like the large Scripto
are twice as expensive). The larger igniters take more
abuse than the little ones, and they are much easier to
connect the wires to, since they have nice little copper
tabs that solder easily (they are designed so
soldering isn't necessary, but I solder them anyway for
a more permanent solid connection).
How does it do that?
While perfume (which is mostly alchohol) works pretty well,
the best fuels are hair spray and Binaca. Hair spray has
alchohol in it also, but it also contains large amounts of
propane, butane, and isobutane as propellants (gases under
so much pressure that they are liquids in the can, and turn
to gas at the nozzle). These gases are excellent fuels.
The Binaca is alchohol and isobutane, and comes in a very
convenient dispenser. It fits easily in a pocket, and delivers
just the right amount of fuel in a single push of its button.
(The hair spray keeps spraying, so it is harder to get just
the right amount).
In order to make an explosion, you need a flamable gas, oxygen,
and a source of heat to start things off. Solids like candle
wax and liquids like alchohol only burn when they are heated
enough to become gases. Then they need a little more heat to
get them to break their chemical bonds so they can combine with
By starting off with a gas like propane, or a vapor like that from
alchohol that has been sprayed in a fine mist, we only need a small
spark to start things burning.
The Film Can Cannon can only hold a small amount of air and fuel
mixture, so it is safe to fire off in the house. The plastic can
is soft and light, and can land on people without disturbing their
hairdo. But it takes off rather quickly, and it is not recommended
to have your head in the way during a launch.
The amount of air required to be mixed with the fuel will vary with
which fuel is used. The ratio of air to fuel (called surprisingly
enough, the 'fuel-air ratio') must be just right for some fuels.
Other fuels (such as hydrogen) have a wide range of ratios which will
Hydrogen will burn in air at concentrations ranging from 4% to 75%
by volume. Methane (natural gas) burns at 5.3% to 15%. Propane
burns at 2.1% to 9.5%. Isobutane burns at 1.8% to 8.4%.
Hydrogen will explode in air at ratios of
13% to 59%. Methane explodes at a much narrower range between 6.3%
and 14% (ratios are fuel to air).
It is easy to see how too little fuel will result in no explosion.
But the ratios we saw in the preceding paragraphs show that the
problem is more likely to be too much fuel. If your can
won't go Bang!, try lifting it off the pad and putting it
back. This will allow a little more air in, and you will probably
get a bang out of the results.
As the fuel-air mixture burns, energy is released by the formation
of chemical bonds between the oxygen in the air and the carbon and hydrogen
in the fuel. This energy heats up the gasses that result from the
burning. The gases are water vapor (H2O) and carbon dioxide
(CO2). Since they are hot, they expand. The expansion pushes
on all sides of the can and its lid. The can and the lid separate
quickly, and the can goes skyward.
A more detailed explanation of exactly how the expanding gases cause
the can to move can be found in the section on
and the section on
How does the igniter work?
The igniter is a piezoelectric generator. The word
piezo comes from the Greek work for press.
A piezoelectric substance is something that makes
electricity when you press on it.
The classic example of a piezoelectric substance is a quartz
crystal. Quartz is made up of atoms of silicon and atoms of
oxygen. These atoms are arranged in neat orderly rows. By
carefully cutting the crystal, we can arrange for the rows
of oxygen atoms and silicon atoms to be parallel to the cut
surfaces, as in the following diagram:
When pressure is applied to the crystal, the negatively charged
oxygen atoms move relative to the positively charged silicon
atoms. This causes electrons in the metal contacts to move,
The piezoelectric material in the igniter is not quartz, but
is instead a man-made ceramic that has been formed under a high
voltage electric field to align the electric charges in it.
These man-made ceramic piezoelectric materials can generate very
The igniter holds this ceramic element in a plastic case, with a
steel hammer attached to a spring and a catch. As you push down
the plunger, the spring is compressed until it hits the catch, which
releases the spring, pushing the hammer quickly down on the ceramic.
The electricity runs through the wires to the spark gap, which it
jumps across, igniting the fuel-air mixture.
The Mark II Film Can Cannon
I was at an upscale lumber yard when I found these little
wooden spoked wheels. I just had to build a film can
cannon with these wheels.
A little walnut, a little rosewood, a little clever
drilling to hide the wiring, and we have the Mark II
Film Can Cannon.
This cannon is angled at 45 degrees to maximize the
distance traveled horizontally (although due to wind resistance
that angle is not exactly optimal).
Of course, this cannon fires its entire barrel, but
I can live with that.
These are so popular I've had to make several to keep
up with the demand. The photo above is either the
fourth or the fifth, I've lost track.
Film Can Cannons
have become so popular that we have
set up an
where we offer kits (and fully assembled
cannons) for sale. You can buy igniters in 10 packs (cheaper and
easier than taking apart lighters if you're making several
cannons) and we even have deluxe model cannons made of exotic
See all of these
in our catalog.
Next: More thermodynamic toys...
For more information on thermodynamics, see the
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Simon Quellen Field