fire from mirrors

[Brocktor]

i watched a program on bbc 2 this morning about historic inventions. one device was 124 small hexagon shaped polished steel mirrors all on a board. when they reflected the sun onto a piece of wood it set alight - this was done with in the UK and the sun did not look at its strongest.

anyone know how to do this on a smaller scale?

 

[spamel]

Use a parabolic mirror, or more commonly known as a shaving mirror! Also the underside of a coke can, polished up to a smooth shiny surface, will do the same thing.

 

[loz.]

i watched a program on bbc 2 this morning about historic inventions. one device was 124 small hexagon shaped polished steel mirrors all on a board. when they reflected the sun onto a piece of wood it set alight - this was done with in the UK and the sun did not look at its strongest.

anyone know how to do this on a smaller scale?

They tried this on mythbusters with no sucess, although with lots of large mirrors

 

[spamel]

They tried this on mythbusters with no sucess, although with lots of large mirrors

They did get success with the parabolic mirror though, and I tried it earlier in the summer and it works quickly!!

 

[loz.]

They did get success with the parabolic mirror though, and I tried it earlier in the summer and it works quickly!!

was that in same episode ? dont recall ! but then again - i'm getting old ! ( and my interest lulled at the lack of explosives ! )

 

[spamel]

No, they did a competition where members of the public made their death rays and brought them onto the show. They then had to compete against each other, one of my favourite episodes because two gorgeous lasses are on it aswell as Kari Byron, Grrrrrrr!

 

[gregorach]

They did a couple of revisits on that one... Adam's big array was far too bendy.

It's certainly possible to set fire to things with mirrors. Whether it's possible that Archimedes actually did it during the siege of Syracuse is another question... And whether he actually did do it is a third.

What really annoyed me on that one was that they concluded, straight off the bat, that it's impossible to accurately aim a mirror by hand over a distance of more than a few feet. I'm sure this came as a big shock to anybody who's competent in the use of hand signal mirrors...

I presume that the show that started this thread off was Adam Hart-Davis' "What The Ancients Did For Us", which also annoys me because he states that Archimedes did do it, despite the total lack of any contemporary accounts and the highly questionable nature of the claim.

Yes, I am a curmudgeon.

 

[loz.]

Kari Byron, Grrrrrrr!

I concur !

 

[loz.]

What really annoyed me on that one was that they concluded, straight off the bat, that it's impossible to accurately aim a mirror by hand over a distance of more than a few feet. I'm sure this came as a big shock to anybody who's competent in the use of hand signal mirrors...

Hi,

Im not sure it was simple as AIMING, but more an issue of focus over the distance, with the ability to create such a large perfect concave and calulate required curves etc to get a focal point at where the ship/raft was ??

then again i failed physics....

 

[gregorach]

No, I was just refering to their initial test where they all got a flat mirror each and tried to aim them at a point on the wall, from which they concluded that a bunch of people with mirrors can't aim them accurately, which was the excuse for building an array.

Sure, getting a good focus at a long focal length is very tricky with a parabolic mirror.

 

[rich59]

I have a theoretical design for a parabolic mirror made from water. Sounds simple so far, but it gets more complicated the further you go into the design.

First make your water into a parabola shape. Actually not has hard as it sounds. If you spin water it makes a parabola shape if no resistance on the out edge. So my idea is to spin a partially filled plate floating in a bucket and keep it spinning steadily with a jet of water from a hose or something. OK, so that gives you a parabola but only a fraction of the light is reflected, I don't know how much.

Next you need some vertical sunlight. So now you need a large mirror suspended over your bucket to direct the light down.

In theory it will work if the diameter of the spinning water surface is large enough.
I'll try it some day.

 

[woodwalker]

These people certainly seem to have got it to work - check this link out:
http://www.solardeathray.com/
Unfortunately they aren't giving out any plans

However they don't seem to be too hard to make.

cheers woodwalker

 

[Bushman_Brett]

The water parabole has been done with clear plastic baloons i forget where i saw it but it is on the net

As for the death ray, iv done a bit of research and trial building on this one and it works for sure although the parabol will make more use of the power per area, if you check around on the net you can find the KW power of the sun per sq meter and its a fair amount. My personal project involves a fair few mirrors, a boiler and as soon as i find one a steam engine, only problem is the boiler construction and keeping it from getting to hot and melting through (some of the testing is done in the canarie islands). Anyways its all good fun just like making water burn and implode

 

[Martyn]

anyone know how to do this on a smaller scale?

I cant remember the name of it offhand, but there is a commercially available, small parabolic firestarter. Though how good the thing is in the UK would be a big issue I would think.

 

[Martyn]

Aha, here we go....

The Solar Spark Lighter

http://store.sundancesolar.com/sospliposiso.html

 

[Brocktor]

interesting... i have never seen this before - a magnifying glass is the closest thing i have seen. they had big versions of what i explained earlier 100's of years ago to burn holes in enemy ships and sink them.

 

[Bushman_Brett]

a cheaper option

http://wildwoodsurvival.com/survival/fire/cokeandchocolatebar/

 

[ilan]

i have a magnifing mirror 8ins dia which i have started fires with in the hight of summer

 

[Glen]

On a somewhat grander scale
http://www.stirlingenergy.com/

 

[Graham_S]

i've got one of those solar spark ligters. they're very good.
another method is to get a big torch (one of those million candlepower jobs you get for a few quid in the garage with your petrol is good)
take the reflector out, remove the bulb and use that. just push your tinder through the hole the bulb fits into and move it back and forth until you hit the focal point.