It makes a great science project. Unlike most, you'll actually get some use out of it afterwards. Some of the principles it demonstrates are:

1. Optics: focusing parallel rays of incident light through the use of a parabolic mirror.

2. Energy transformation: light to heat

3. Renewable energy: solar power

Materials:

2 - 2x4 cut to 17" length (ends)

2 - 1/2" plywood cut to 1ft x 4ft (sides)

1 - 5/16" plywood cut to 18" x 4ft (backing)

1 - sheet of silvered mylar or tin foil cut to 17" x 4ft (mirror)

2 - 1/2" plywood cut to 3/4" x 12" (skewer mounts)

1 - 1/4" wooden dowel with a sharpened point (skewer)

Plus miscellaneous fasteners and spray adhesive

Tools:

Jig saw

Power drill

Drill bits

Screw driver bits

**Signing Up**

## Step 1: Optics 101: Focussing light with a parabolic mirror

The formula for a parabola is:

Y = X

^{2}/(4F)

Where:

X is the horizontal distance from the origin

F is the focal distance (a design choice)

Y is the vertical distance to the curve at any point X

See the image below for a graphical representation.

## Step 2: Cut the sides to a parabolic profile

Y = X

^{2}/72 (where both X and Y are in inches).

I used a spreadsheet to calculate and plot the value of Y for all values of X from -24" to +24".

Y is the same whether X is positive or negative, so I've only shown Y values for X values from 0 to 24 here). Those values are:

X - Y

0 - 0.000

1 - 0.014

2 - 0.056

3 - 0.125

4 - 0.222

5 - 0.347

6 - 0.500

7 - 0.681

8 - 0.889

9 - 1.125

10 - 1.389

11 - 1.681

12 - 2.000

13 - 2.347

14 - 2.722

15 - 3.125

16 - 3.556

17 - 4.014

18 - 4.500

19 - 5.014

20 - 5.556

21 - 6.125

22 - 6.722

23 - 7.347

24 - 8.000

Draw a grid with 1" spacing on one of the 2 sheets of plywood to be used for the sides. similar to that shown below, and plot the curve. Then simply cut along the curve with a jig saw. You can repeat the process on the second sheet or use the first sheet as a template to trace the curve onto the second (or if you are a brave enough, you can attempt to cut them both at the same time... clamp them together well first).

## Step 3: Assemble the parts

Draw a pencil mark on each side at the origin of the parabolic curve.

Draw a line across the center of the backing (the 18" x 4ft sheet of 5/16" plywood) dividing it into two halves (each 18" x 2ft).

Line up the backing with the sides and put a small screw (I used 3/4" #6 screws) through the center line on the back and into each side at the origin marks you made.

Every 6" or so, put another screw through the backing into each side until you have attached the backing to the sides along the entire length of the sides (bend the backing to match the shape of the sides as you go).

Now glue your mylar or tin foil to the inside surface of the backing. Spray adhesive works well. Try to smooth out the surface as much as possible. Any little wrinkles will reduce the efficiency of the cooker. You can see that I wasn't that careful myself.

## Step 4: Attach the skewer mounts

I did this with a couple pieces of 1/2" plywood cut to about 12" x 3/4", but you can use any material you have available. Screw these pieces to the center of each side so they stick out a little over 6" to hold the skewer. If you want you can make them pivot so they can fold out of the way when not in use.

Make a notch in the end of one skewer holder, and drill a hole in the end of the other. The sharpened end of the skewer will stick into the hole. The other end of the skewer will rest in the notch.

Now you are completely assembled and ready to cook.

## Step 5: Cook your weiner.

The easiest way to tell when it is aimed correctly is to look at the shadows cast by the two skewer mounts. When you can see a shadow cast on the inside surface of each side, then the cooker is aimed in the right direction. When both shadows pass through the origin of your parabola then the cooker is tilted to the right height. You will have to adjust the cooker from time to time as the sun moves (or rather as the Earth rotates).

Shove your skewer through up to 3 hot dogs, stick it on your skewer mounts and wait.

Be patient. My experience with solar cooking is that it looks like nothing is happening (you don't hear anything or see anything) until suddenly, your hot dog starts blistering and/or your skewer catches on fire. It takes a bit of practice to learn when you should rotate your skewer.

I find it takes about 6 minutes to cook ordinary weiners, and 8-10 minutes to cook the large smokies you've seen in these images. I try to rotate the skewer about once every couple minutes.

For more ways you can reduce your environmental impact and save money, visit my website www.IWillTry.org. Thanks for reading.

I've always had trouble with the math... Okay, maybe I haven't even tried. But when I seen the pic in the link above - making parabolic curves immediately became clear to me. The distance to the focal point to the bottom of the curve needs to be mirrored for the flat/base line below, then finding every other point along the curve is a matter of finding the right intersecting radii. It's a bit more complicated than that - but it can be figured out.

To scale it up - you can keep doubling the distances of the points on the different angles.

Then there is using wax paper. Put a point where the focus will be, then a line twice the distance of where the base of the curve (or trough) will be. Then fold the paper over to where the line meets the point, and keep folding along the line to get the full curve.

One thing you could try that *may* help is placing a transparent shell around the hot dog to help insulate it (the air inside is what actually insulates it. the shell just hold the air in place). Cut the tops off two 1 liter pop bottles, drill a hold in the bottom of each to fit over your skewer, and slide the two pieces over each end of your hot dog so they meet in the middle. Might help, but probably you just need a bigger reflector. Also, if it gets too hot that you smell plastic fumes, you may not want to eat the results ;-).

^{2}20cm = 0.2m.

0.2m * 1m = 0.2m

^{2}0.2m

^{2}* 1000W/m^{2}= 200W_{its reaLly GENUS!!!}havebeen done before , but the big one in the background is really, really interesting. Could you give us one on that one, including the metrics by which you came up with the 2KW of heat? That is daggne interesting!I am looking at designing a simply tracker for them and a base for the leg keep a look out on http://www.bonzabuy.com.au

Also might be keen on linking to this for people who want to try & build a parabolic for themselves - thanks again!