Splitting Water: Electrolysis Experiment

Is it possible to break water? In a sense, that's what electrolysis does. Electrolysis uses electricity to split water into its two ingredients: hydrogen and oxygen. Try it out with a battery and a couple pencils! (Adult supervision required.)

Materials

• 6-volt or 9-volt battery
• Two alligator clip leads or insulated wire
• Beaker or glass
• Piece of thin cardboard or cardstock
• Two #2 pencils 

What to do:

1. Fill the beaker or glass with warm water.
2. Carefully remove the erasers and metal sleeves so you can sharpen both ends of each pencil.
3. Cut a piece of the cardboard to fit over the beaker, then punch two holes in the center of the cardboard about an inch apart. Push the pencils through the holes and set them in the glass. They should extend into the water, but not touch the bottom of the glass. The cardboard will hold them in place.
4. Connect each pencil to the battery with an alligator clip lead attached to the exposed graphite (pencil lead). If you don't have alligator clip leads, use two lengths of wire and strip an inch of insulation off each end. Wrap the wire around the graphite of each pencil and connect the wires to the battery. You may need to use tape to hold the wires in place.

What's happening?

As soon as you connect the wires to the battery, you will see bubbles appearing around each of the pencil tips in the water and floating upward. Those bubbles are the components of water—hydrogen and oxygen gas—that have been split apart by the electricity as it travels through the water from one pencil to the other. The pencil attached to the negative terminal of the battery collects hydrogen gas while the one connected to the positive terminal collects oxygen. Does one pencil collect more bubbles than the other? Which one? Why do you think this is?

A SIMPLE CRYSTAL RADIO

A crystal radio is the distilled essence of a radio. It has very few parts, it needs no batteries or other power source, and it can be built in a short time out of things you can find around the house.

The reason a crystal radio does not need any batteries is the amazing capabilities of the human ear. The ear is extremely sensitive to very faint sounds. The crystal radio uses only the energy of the radio waves sent by radio transmitters. These radio transmitters send out enormous amounts of energy (tens of thousands of watts). However, because they are usually far away, and we have at most a few hundred feet of wire for an antenna, the amount of energy we receive with the crystal radio is measured in billionths of a watt. The human ear can detect sounds that are less than a millionth of even that.

We are going to launch right into this chapter by building a working radio using parts that we buy at stores like Radio Shack or through mail order. We will try to use common household objects when we can, but our emphasis will be to quickly put together a radio that works.

Later we will learn more about radios by looking at even simpler versions that might not work as well as our first radio, but can show the important radio concepts more easily, because they have fewer parts.

Then we will improve our radio, making it louder, making it receive more stations, and making it look real nice.

Lastly, we will build each part of the radio from scratch, using things we find around the house. This will take a lot longer than our first radio, but it can be done by replacing store-bought parts one at a time, so we always have a working radio.

Our first radio

For our first radio, we will need these parts:

• A sturdy plastic bottle.
  I have used the plastic bottle that hydrogen peroxide comes in, or the bottles that used to contain contact lens cleaner. They are about three inches in diameter, and 5 to 7 inches long. Shampoo bottles also work, but you will want to get the ones with thick walls, rather than the thin flimsy ones. This will make it easier to wind wire around them.
• About 50 feet of enamel coated magnet wire.
  Most common gauges (wire diameters) will work, but thicker wire is easier to work with, something like 22 gauge to 18 gauge. This can be bought at Radio Shack (part number 278-1345), or you can take apart an old transformer or electric motor that is no longer needed. You can also use vinyl coated wire such as Radio Shack part number 278-1217, which in some ways is easier to use than enamel coated wire (it is easier to remove the insulation).
• A Germanium diode.
  Most stores that sell electronic parts have these. They are called 1N34A diodes (Radio Shack part number 276-1123). These are better for our radio than the more common silicon diodes, which can be used but will not produce the volume that Germanium diodes will. We also carry it in our catalog.
• A telephone handset.
  You listen to this radio just like you listen to the phone. If you have an old telephone sitting around, or can find one at a garage sale, you are set. Or you can buy the handset cord (Radio Shack part number 279-316) and borrow the handset from your home phone (using it for the radio will not harm it).
• A set of alligator jumpers.
  Radio Shack part number 278-1156, or you can find them anywhere electronics parts are sold.
• About 50 to 100 feet of stranded insulated wire for an antenna.
  This is actually optional, since you can use a TV antenna or FM radio antenna by connecting our radio to one of the lead-in wires. But it's fun to throw your own wire up over a tree or on top of a house, and it makes the radio a little more portable.

Use a sharp object like a nail or an icepick to poke four holes in the side of the bottle. Two holes will be about a half an inch apart near the top of the bottle, and will be matched at the bottom of the bottle with two more just like them. These holes will hold the wire in place.

Thread the wire through the two holes at the top of the bottle, and pull about 8 inches of wire through the holes. If the holes are large and the wire is loose, it is OK to loop the wire through the holes again, making a little loop of wire that holds snuggly.

Now take the long end of the wire and start winding it neatly around the bottle. When you have wound five windings on the bottle, stop and make a little loop of wire that stands out from the bottle. Wrapping the wire around a nail or a pencil makes this easy.

Continue winding another five turns, and another little loop. Keep doing this until the bottle is completely wrapped in wire, and you have reached the second set of holes at the bottom of the bottle.

Cut the wire so that at least 8 inches remains, and thread this remaining wire through the two holes like we did at the top of the bottle. The bottle should now look like this:

Now we remove the insulation from the tips of the wire, and from the small loops we made every 5 turns (these loops are called 'taps'). If you are using enameled wire, you can use sandpaper to remove the insulation. You can also use a strong paint remover on a small cloth, although this can be messy and smelly. Don't remove the insulation from the bulk of the coil, just from the wire ends and the small loops. If you are using vinyl coated wire, the insulation comes off easily with a sharp knife.

Next we attach the Germanium diode to the wire at the bottom of the bottle. It is best to solder this connection, although you can also just twist the wires together and tape them, or you can use aligator jumpers (Radio Shack part number 278-1156) if you are really in a hurry.

Cut one end off of the handset cord to remove one of the modular telephone connectors. There will be four wires inside. If you are lucky, they will be color coded, and we will use the yellow and black wires. If you are not lucky, the wires will be all one color, or one will be red and the others will be white. To find the right wires, first strip off the insulation from the last half inch of each wire. Then take a battery such as a C, D, or AA cell, and touch the wires to the battery terminals (one wire to plus and another to minus) until you hear a clicking sound in the handset earphone. When you hear the click, the two wires touching the battery are the two that go to the earphone, and these are the ones we want.

The 'wires' in the handset cord are usually fragile copper foil wrapped around some plastic threads. This foil breaks easily, sometimes invisibly, while the plastic threads hold the parts together making it look like there is still a connection. I recommend carefully soldering the handset wires to some sturdier wire, then taping the connection so nothing pulls hard on the copper foil.

Attach one handset wire to the free end of the Germanium diode. Solder it if you can.

Attach the other wire to the wire from the top of the bottle. Soldering this connection is a good idea, but it is not necessary.

Now clip an alligator jumper to the antenna. Clip the other end to one of the taps on the coil.

Clip another alligator lead to the wire coming from the top of the bottle. This is our 'ground' wire, and should be connected to a cold water pipe or some other metal object or wire that has a good connection to the earth.

At this point, if all went well, you should be able to hear radio stations in the telephone handset. To select different stations, clip the alligator jumper to different taps on the coil. In some places, you will hear two or more stations at once. The longer the antenna is, the louder the signal will be. Also, the higher you can get the antenna the better.

Now that your radio works, you can make it look better and be sturdier by mounting it on a board or in a wooden box. Machine screws can be stuck into holes drilled in the wood to act at places to attach the wires instead of soldering them. A radio finished this way looks like the following photo. Note the nice little touch of using brass drawer pulls on the machine screws to hold the wire.

Building radio-10 minutes

For our 10 minute radio, we will need these parts:

• A ferrite loop antenna coil
  In our other crystal radios we wound the coil by hand. In this project we use a much smaller coil with a ferrite rod inside, from our catalog. The ferrite rod allows the coil to be smaller, and it can be moved in and out of the coil for coarse tuning.

• A variable capacitor (30 to 160 picofarads)
  We carry this in our catalog. You can also find them in old broken or discarded radios.

• A Germanium diode (1N34A)
  We carry this in our catalog.

• A piezoelectric earphone
  Also in our catalog.

• Two alligator jumper wires
  We use alligator jumper wires here for convenience. They are used to connect the ground and antenna wires to a good ground and a long wire antenna. We carry these in our catalog.

• About 50 to 100 feet of stranded insulated wire for an antenna.
  This is actually optional, since you can use a TV antenna or FM radio antenna by connecting our radio to one of the lead-in wires. But it's fun to throw your own wire up over a tree or on top of a house, and it makes the radio a little more portable.

• A block of wood or something similar for a base

Click on photo for a larger picture

You can see from the photo how simple this radio is, and why it can be put together in a very short time.

The black painted wire from the ferrite loop is soldered to the center lead of the variable capacitor. The unpainted wire is soldered to the rightmost lead of the variable capacitor.

The germanium diode is soldered to the rightmost lead of the variable capacitor.

One of the piezoelectric earphone wires is soldered to the free end of the germanium diode. The other is soldered to the center lead of the variable capacitor.

The red painted wire of the coil is attached to the long wire antenna with an alligator clip lead.

The green painted wire of the coil is attached to a good ground (such as a cold water pipe) using another alligator clip lead.

That's it -- you're done!

Click on photo for a larger picture

How does it work?

We will start the tuning with the variable capacitor set in the middle of its range, neither all the way clockwise, nor all the way counter clockwise.

With the earphone in your ear, slowly move the ferrite rod into the coil, listening for radio stations.

With a long antenna, you can tune several radio stations. In some areas, one or two stations will be so close or so powerful that they overwhelm all the others, and you will only hear those one or two stations.

How does the ferrite change the frequency?

The ferrite rod increases the inductance of the coil. In our other (hand-wound) coils, we increased the inductance by winding some more loops, or by using a "tapped" coil, and selecting a tap that was farther down the coil.

As the ferrite rod is inserted into the coil, more of the coil is affected by the ferrite, and so the inductance increases. Increasing the inductance moves the frequency lower. This allows us to hear stations "lower on the radio dial".

Ferrite is used because it is magnetic, like iron or steel, but it is not a conductor of electricity. If it were conductive, the coil would induce "eddy currents" in it, and some of the energy would be lost heating up the core. Because ferrite is not a conductor, we can use its magnetic properties to change the inductance of the coil, without losing volume.

If you have a long antenna, a good ground, and you are not too close to a strong station, the variable capacitor will help in fine tuning the stations.

There are actually two coils of wire wound around the ferrite rod. The large coil is connected to the variable capacitor. The small coil is connected to the antenna and ground.

This arrangement allows the radio to be more selective, so that strong stations don't drown out the weak ones. Really strong local stations will still overwhelm the more distant stations, however.

If you have no strong local stations, you can make the stations you receive sound louder by connecting the antenna and ground directly to the large coil. Connect the antenna to the center lead of the variable capacitor, and the ground to the rightmost lead of the variable capacitor. The stations will be louder, but they will most likely all be heard at once, since you radio will be less selective in tuning out adjacent stations.

A SIMPLE SPECTROSCOPE

  A spectroscope is a device that lets us find out what things are made of. It works by taking light and splitting it up into its component colors. Different elements make different colors when they glow. We can make objects and gasses glow by heating them up in a flame, or by passing electricity through them. The spectroscope spreads out the colors of the light, and we can identify the elements by the bright lines we see in the spectroscope.

Fluorescent light

The photograph above was made using the homemade spectroscope we will make in this project. You can see a bright green line, a bright blue-purple line, and a fainter orange line. These lines tell us that the element mercury is making some of the light. The light is coming from a fluorescent light bulb, and these bulbs work by heating up mercury until it glows. The blue-purple light from the mercury (and some invisible ultraviolet light just beyond the blue-purple line) then makes the white phosphor coating on the inside of the glass tube fluoresce bright white.

How to make a spectoscope

Click on photo for larger picture

What we will need:

1. A CD or DVD that can be sacrificed to this project. We won't damage it, but getting it back will involve destroying our spectroscope. Old software CDROMs work great, and some can be had for free from internet service providers like AOL.

2. A cardboard box. An 8 inch cube works fine, but any size that can hold a CD or DVD disk will do.

3. Two single edged razor blades. These can be found in paint or hardware stores.

4. A small cardboard tube, the kind used as a core to wrap paper on.

5. Some cellophane tape.

6. Some aluminum tape (found in hardware stores), or some aluminum foil and glue.

Click on photo for larger picture

Our spectroscope has three main parts. There is a slit made from two razor blades, a diffraction grating made from a CD disk, and a viewing port, made from a paper tube.

To make sure that all three parts are lined up properly, we will use the CD disk as a measuring device, and mark the spots where the slit and the viewing port will go.

Set the CD disk on top of the box, about a half inch from the left edge, and close to the box's bottom, as shown in the photo. Use a pen to trace the circle inside the CD disk onto the box. This mark shows us where the paper tube will go.

Click on photo for larger picture

Now place the paper tube on the box, centered over the circle we just drew. Draw another circle on the box by tracing the outline of the paper tube.

Click on photo for larger picture

Move the paper tube over a little bit. A half-inch is probably fine -- in the photo I placed it much farther to the right than necessary, but the aluminum tape covered up the mistake nicely. Trace another circle around the paper tube. These circles will tell us where to cut the box.

Click on photo for larger picture

Now cut an oval out of the box with a sharp knife. The oval will allow the paper tube to enter the box at an angle.

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The next step is to make the slit. Turn the box one quarter turn so the oval we just cut is to the right. Using the CD disk again, draw another small circle close to the left side of the box.

The slit will be on the far left of the box. Cut a small rectangle out of the box at the height marked by the small circle we made with the CD disk. The rectangle should be about a half inch wide, and two inches high.

Click on photo for larger picture

Carefully unwrap the two razor blades, and set them over the rectangular hole. Make their sharp edges almost touch. Tape the razor blades to the box, being careful to leave a gap between the sharp edges that is nice and even, and not wider at the top or bottom.

Click on photo for larger picture

Next, set the box right-side-up, with the slit towards you. Now tape the CD disk onto the back wall of the box. The rainbow side should face you, with the printed side touching the cardboard. The photo shows the disk a little too far to the left. The left edge of the disk should be the same distance from the left of the box as the slit is.

Click on photo for larger picture

Now seal up any places on the box where light might leak in. Use the aluminum tape for this. You can also use aluminum foil for this purpose if you don't have any aluminum tape.

Click on photo for larger picture

The last step is to use the aluminum tape to attach the paper tube. The aluminum tape will make a light-tight seal around the tube. To make sure the angle is correct, hold the slit up to a light, and look through the paper tube, adjusting it until you can see the full spectrum from red to purple.

That's it! We are ready to use the spectroscope.

How to use the spectoscope

Hold the slit up to a source of light. An incandescent light will show a simple spectrum with no bright lines. This is because the light comes from a hot solid (the tungsten filament in the light bulb).

Hot gasses will produce light that is made up of only a few colors. The spectroscope will spread these colors out, so we can see them individually.

Neon light bulb

The photo above shows the light from a neon light bulb. We can see that the hot neon gas is made of several colors, but they are mostly in the red and orange parts of the spectrum.

Light is made up of waves, and each different wavelength is a different color. The neon light is showing waves of these lengths:

• 540 nanometers (very faint) green
• 585 nanometers yellow
• 588 nanometers yellow
• 603 nanometers orange
• 607 nanometers orange
• 616 nanometers orange
• 621 nanometers red-orange
• 626 nanometers red-orange
• 633 nanometers red
• 638 nanometers red
• 640 nanometers red
• 650 nanometers red
• 660 nanometers red
• 692 nanometers red
• 703 nanometers red

Red light-emitting-diode (LED)

A red LED makes light, but there is no hot gas, so it has a continuous spectrum.

Green light-emitting-diode (LED)

Some green LEDs look very green. Others look more like a yellowish-green. By looking at their spectrum, as in the photo above, we can see that the yellow-green LEDs have a lot of green, but also some yellow, orange, and red.

Laser diode

The LEDs we looked at had broad spectra. Their light consists of many different wavelengths. A red laser diode has a much narrower spectrum. It has only a few different wavelengths, and is said to be monochromatic, meaning "one color".

White light-emitting-diode (LED)

A white LED is actually a blue LED and a phosphor. It works in a way similar to the fluorescent light bulb, where the blue light excites the phosphors to make a white glow.

We can see the broad spectrum in the spectroscope. The dark band in the photo is the bare spot on the CD disk close to the center. The spectrum is so broad that it covered the entire width of the CD.

The photos above were done using a spectroscope made from an audio CD. The photo below was done using a DVD, which has lines that are closer together. The closer lines cause the spectrum to spread out a little more.

Mercury vapor spectrum in fluorescent light

The spectrum is not quite spread enough to show the two orange lines as separate lines. A diffraction grating with finer lines would show finer distinctions, allowing us to distinguish elements better.

A spectroscope made from a cereal box

A CD is an example of a reflection diffraction grating. But you can also find transmission diffraction gratings, that you look through, instead of look at.

A fairly common toy using a transmission grating is a pair of paper "rainbow glasses". These are often sold at carnivals or fairs, or at fireworks displays. We also carry them in our catalog.

Rainbow glasses -- Click on photo for larger picture

These glasses typically make rainbows up and down as well as left and right.

Making a spectroscope with rainbow glasses is very simple. We use most of the same materials and techniques as we did for the CD spectroscope.

Click on photo for larger picture

We need a cereal box, some aluminum tape, two single edged razor blades, and a pair of rainbow glasses. You can substitute aluminum foil and glue for the aluminum tape.

Click on photo for larger picture

We will only need one side of the rainbow glasses. Or you can make two spectroscopes. Cut the plastic material from the glasses, leaving a paper border to keep it easy to handle.

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Cut a hole in the top of the cereal box, on the right side, just big enough for the plastic window of the rainbow glasses. The hole should be smaller than the paper border around the plastic, so the plastic doesn't fall into the box.

Tape the plastic window over the hole, and seal out any extra light with the aluminum tape.

Cut a thin rectangle out of the bottom of the cereal box, opposite the plastic window. This is where our slit will be.

Click on photo for larger picture

Seal all around the slit with aluminum tape, to keep out stray light.

That's it! You now have another spectroscope!

Click on photo for larger picture

Hold the slit up to a fluorescent light bulb, and look into the plastic window. You will see something like the photo below.

Click on photo for larger picture

The photo above was taken using a very fast shutter speed, since the image was quite bright. The fast shutter speed only captures the primary rainbows -- those closest to the slit.

Click on photo for larger picture

However, there are also secondary rainbows farther from the slit, and they are spread out more, allowing us to more easily see the bright lines in the spectrum. The second photo used a slower shutter speed, and shows the secondary spectra, while washing out the primary.

Click on photo for larger picture

The photo above was taken from about 20 feet away from a bright (90 watt) fluorescent bulb. The lines are bright and nicely separated. With a narrower slit, I suspect the green line would resolve into two lines, and the orange line would become five or six separate lines.

I have digitally cropped out the extra spectra above and below the horizontal band.

Higher resolution

You can get high efficiency holographic diffraction gratings from scientific supply stores. We also carry them in our catalog. With 1000 lines per millimeter, these gratings separate the spectral lines well, and the lines are still quite bright.

I replaced the grating in the cereal box spectroscope with one of these high resolution gratings, and you can now clearly see several lines in the yellow and orange that were smeared together in the earlier version.  

COOK WITH THE SUN HEAT

 We will now show you how to make a powerful solar concentrator that can cook four or five hotdogs in minutes.

The Solar Hotdog Cooker is made out of a thin (1/8 inch thick) plastic mirror that can be found at plastic shops and glass stores (although it may have to be special ordered at some stores). The plastic is bent into the shape of a parabola, so that the sun's rays are collected over an eight square foot area, and focused in a thin line. The hotdogs are roasted on a spit placed at the focus, and turned every once in a while to prevent them from burning.

Click on picture to see a larger image

(( IF YOU NEED ANY HELP THEN MAIL TO ME   salman_ln@hotmail.com  ))

Materials

For the solar cooker you will need:

1. Two pieces of plywood, 1/2 inch thick, 2 feet wide and 4 feet long.

2. Two pieces of lumber, (2x4) 1 1/2 inch thick, 3 1/2 inch wide, and 8 feet long.

3. 16 wood screws, 2 inches long.

4. One stiff steel wire, 3 feet long.

5. 92 small nails or wooden pegs, about an inch long.

6. One plastic mirror, 1/8 inch thick, 2 feet wide, 6 feet long (although 5 1/2 feet long might work better).

7. A drill and a bit that matches the diameter of the 92 small nails or pegs. A larger bit (over 1 inch wide) is needed for the food hole.

Assembly

Place the two sheets of plywood together, one on top of the other. Using a tape measure and a carpenter's square, mark off where the holes will be drilled for the mirror supports (the 92 small nails or pegs).

All holes are drilled completely through both sheets of plywood. The holes are to be drilled according to the following table:

Inches from left Inches from bottom 0 22.16 2 18.94 4 16.00 6 13.34 8 10.96 10 8.86 12 7.04 14 5.50 16 4.24 18 3.26 20 2.56 24 2.00 28 2.56 30 3.26 32 4.24 34 5.50 36 7.04 38 8.86 40 10.96 42 13.34 44 16.00 46 18.94 48 22.16

Spacing for drilled holes A second row of holes is drilled above these, separated by the thickness of the mirror.

Next drill a set of holes above the first set, about a third of an inch above the first set of holes. The first set of holes will eventually have 23 of the small nails placed in each side, to hold the mirror up. The second row will also have 23 small nails pushed in, this time to hold the mirror in place from above. The exact spacing is not critical, but you don't want them too close together, or the top nails will hit the mirror instead of resting on top of the mirror.

Next drill eight holes for the screws that will hold the 2x4 lumber in place. The holes are 3/4 inch from the edges of the plywood. On the left and right, a pair are drilled 15 inches from the bottom and 13 inches from the bottom. At the bottom, a pair are drilled 10 and 12 inches from the left, and the last pair is 36 and 38 inches from the left.

The focus of the parabola is 9.14 inches from the bottom, and 24 inches from the left. Drill a hole that is the same diameter as the spit wire, or a little bit larger. This hole should go through both sheets of plywood.

Just above one of the focus holes, drill a large hole in one plywood sheet, just touching the hole for the spit. This large hole will accomodate the food (hotdogs or kebabs), so it should be at least an inch in diameter, but three or four inches would be better. The spit with the food on it will be inserted into this hole, and the spit will then drop into the much smaller hole at the focus, to keep the spit in exactly the right place.

Cut four pieces from the 2x4 lumber. Each piece should be exactly 2 feet long.

Using the 2 inch long screws, screw the 2x4 pieces to one of the plywood sheets, centering each pair of screws in the end of each piece of 2x4. The result should look something like the legs of a small table.

Attach the second plywood sheet to the other end of the 2x4 pieces.

Click on picture to see a larger image

The photo above shows the back side of the cooker, where the 2x4 spreaders can be seen. Note also the remaining length of 2x4 is used as a support (more about that later).

Next push 46 of the small nails into the bottom row of holes.

Now set the mirror onto the top of the cooker, and gently push it down to rest on the nails. Put a pair of nails in the center pair of holes on top of the mirror, then work your way outwards, placing pairs of nails to hold the mirror down. (I used cotton tipped wooden swabs in the picture because they photograph better than nails.)

The last step is to place a few screws in the remaining long piece of 2x4, leaving the head of the screws sticking an inch or two out of the wood. These will act as supports to hold the cooker so it is tilted toward the sun.

The spit is formed from the 3 foot piece of wire. A coat hanger can be used, but wires that thin tend to sag in the middle when burdened by a few hotdogs. A thicker, stiffer wire is better.

To make it easier to turn the food, a crank is formed by bending the wire at one end as shown in the labeled photo.

Cooking with the sun

Carefully poke the 3 foot wire spit through the hotdogs or kebabs. Try to center the food on the spit, so the food will rotate when you rotate the spit, instead of slipping to keep the heavy part down.

Insert the spit through the food hole, and insert the far end of the wire into the small focus hole in the far plywood sheet.

Rest the near end of the spit in the small focus hole at the bottom of the food hole.

Align the solar cooker with the sun. Start with the cooker flat on the ground, then turn it until it is parallel with your shadow. The sun will just barely graze both of the plywood sheets when the cooker is aligned properly (this can be seen in most of the photos on this page).

Next tip one end of the cooker up until the shadow of the spit falls directly on the center nail at the bottom of the parabola. This can be clearly seen in the labeled photo.

Hold the remaining scrap of 2x4 up against the back side of the cooker, and mark where a screw should be placed to hold the cooker at the right elevation. Screw the screw into the 2x4, leaving an inch or two sticking out to hold the top 2x4 spreader. If you like, the screw can be placed a little higher up, and the cooker can be adjusted to the exact angle by tilting the support backwards.

Click on picture to see a larger image

When the cooker is adjusted properly, the sun will be focused on the food, making bright lines across it (sunglasses are recommended at this phase). You can see the shadows of the nails on the walls of the cooker. These shadows should all cross at the focus, where the hotdogs are.

The hotdogs can be seen in the mirror, highly magnified. The shadow of the hotdogs can be seen being cast by the mirror onto the back side of the hotdogs in the photo above. In the photo below, the shadows of the nails can be clearly seen, crossing at the focus of the parabola.

Click on picture to see a larger image

The hotdogs will start steaming in less than a minute. The spit should be turned every couple minutes to prevent black lines from being burned into the food (unless you like your hotdogs with black stripes). The hotdogs will be quite hot in about 10 minutes, or burned black all over in about 20 minutes.

Click on picture to see a larger image

Things to notice in the above photo:

1. The shadows of the mirror supports seem to meet at the focus.
2. The shadow of the hotdogs is projected onto the enlarged reflection of the hotdogs.
3. The enlargement of the hotdogs only occurs in their width, not their length, because the mirror is only curved in one dimension.
4. The poor hotdogs have been burned to a crisp (oops...).

How does it do that?

A parabola is a shape with some interesting properties that make it perfect for cooking hotdogs.

The sun is bigger than the earth, and very far away. This means that the sunlight that hits the earth appears to be in parallel rays.

If we had thousands of tiny mirrors, connected by hinges in a line, and we tilted each mirror so it would reflect these parallel rays onto one spot, the mirrors would line up in a parabola.

Mathematically, a parabola is defined as a set of points that are the same distance from both a point (called the focus) and a straight line (called the directrix).

The formula for the parabola used in the solar cooker is

y = 0.035x²+2

I chose this formula so the parabola would be deeply curved, and would fit into the 2 foot by 4 foot plywood sheets. We want the focus to be close to the mirror, so that as the sun moves, the focus does not move very much.

Having the focus close to the mirror is like having the fulcrum of a lever close to one end. The sun end of our lever can move a lot, while the hotdog end of our lever hardly moves at all. This means that we don't have to raise or lower the cooker very often as the sun moves.

The +2 part of the equation says that the bottom of the parabola will be 2 inches from the bottom of the plywood. This gives us room for the 2x4 spreaders, and room to drill the bottom hole for the support nails.

The bottom of the parabola is called the vertex. The vertex is always halfway between the focus and the directrix. The distance from the vertex to the focus is

1

---

0.035

---

4

or about 7.14 inches.

A square meter of the earth's surface gets about 1000 watts of power from sunlight. Our mirror intercepts about 8 square feet of sunlight, or about three quarters of a square meter. This means that our cooker is the rough equivalent of a 750 watt electric stove.