Paper Parabolæ
>that we have
at our various warehouses one eight-foot and one sixteen-foot parabolic
mirror. These were made from fiberglass satellite antennas lined with small
plane mirrors >
Yow! Sixteen-foot, that sounds great. You might not bring down aircraft, but in sunshine the focus must get seriously hot. We made much smaller parabolæ from papier maché, and had huge fun with them. We calculated and cut a half-parabola in 10mm ply, and mounted it, along the axis, to pivot on a piece of metal pipe banged into the ground. Piling rocks and stones around it to just clear when the curve was turned round the axis, we covered the mound thus formed with cement and skimmed it off with the wooden form.
Next day the parabolic cement bump was sanded smooth and given a coat or two of gloss paint. When dry, we smoothed aluminium cooking-foil all over, and burnished it down with the back of a spoon. We then built up, over several days, layers of newsprint papier-maché with a dose of PVA wood glue in, and eventually radial "ribs" made from old rope. The resulting dishes, about 45" in diameter, were flexible but surprisingly strong. The kids used them;
- As umbrellas, when it rained...
- As Frisbees, until they got too bent to fly...
- As shields, wash-basins, for collecting blackberries...
And of course for their original purpose: to concentrate sunlight. The focal length was about six feet, and when freshly-made the hot-spot about 2" - 3" wide. Three dishes together, in bright sunshine, would ignite a paraffin-soaked rag as it fell through the combined focus, and our coffee-pot, already as black as night from having been on the open fire, boiled in a very short time. Dangerous? But of course....He He!
I saw two large parabolæ face-to-face across a very large room at the Science Museum in Kensington, London. The foci were shown by loops on metal poles, the idea being that you could talk into one and hear at the other. Worked surprisingly well, across a noisy room. They had some good things there, a part for kids, the "Launch Pad". I remember a bottle-rocket running on a very long loop of curtain track that was impressive too, giant soap films....have a look at DIY on - http://www.sciencemuseum.org.uk/on-line/launchpad/index.asp
at our various warehouses one eight-foot and one sixteen-foot parabolic
mirror. These were made from fiberglass satellite antennas lined with small
plane mirrors >
Yow! Sixteen-foot, that sounds great. You might not bring down aircraft, but in sunshine the focus must get seriously hot. We made much smaller parabolæ from papier maché, and had huge fun with them. We calculated and cut a half-parabola in 10mm ply, and mounted it, along the axis, to pivot on a piece of metal pipe banged into the ground. Piling rocks and stones around it to just clear when the curve was turned round the axis, we covered the mound thus formed with cement and skimmed it off with the wooden form.
Next day the parabolic cement bump was sanded smooth and given a coat or two of gloss paint. When dry, we smoothed aluminium cooking-foil all over, and burnished it down with the back of a spoon. We then built up, over several days, layers of newsprint papier-maché with a dose of PVA wood glue in, and eventually radial "ribs" made from old rope. The resulting dishes, about 45" in diameter, were flexible but surprisingly strong. The kids used them;
- As umbrellas, when it rained...
- As Frisbees, until they got too bent to fly...
- As shields, wash-basins, for collecting blackberries...
And of course for their original purpose: to concentrate sunlight. The focal length was about six feet, and when freshly-made the hot-spot about 2" - 3" wide. Three dishes together, in bright sunshine, would ignite a paraffin-soaked rag as it fell through the combined focus, and our coffee-pot, already as black as night from having been on the open fire, boiled in a very short time. Dangerous? But of course....He He!
I saw two large parabolæ face-to-face across a very large room at the Science Museum in Kensington, London. The foci were shown by loops on metal poles, the idea being that you could talk into one and hear at the other. Worked surprisingly well, across a noisy room. They had some good things there, a part for kids, the "Launch Pad". I remember a bottle-rocket running on a very long loop of curtain track that was impressive too, giant soap films....have a look at DIY on - http://www.sciencemuseum.org.uk/on-line/launchpad/index.asp
posted by Simeon Lapinbleu at 18:10
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