[tricker2500]

hello

I'm working on this school project, and i want to build a welding mask, which automatically turns then when welding starts.
problem is i don't know how to do this, so now I'm looking for a chemical that once electrified it either goes very dark (so dark you can weld with it) or opaque so you can see through.
-and then switch between the two.

in advance thanks!:)

[Arkcon]

There have been, for a long time now, liquid crystals sandwiched between windows that become opaque at the flip of a switch.  I don't know how cost effective they are.

[Enthalpy]

Liquid crystals are the standard method for welding masks.

You might think at the electrochemical cell I proposed for telephone cards
http://www.google.com/patents/EP0564012A1?cl=fr (must exist in English)
but beware I didn't prototype it, so it would need serious development, and I ignore how many times the cell would work.

[almfranc]

There are also some electrochromic polymers that are being manufactured, if you don't want to go with liquid crystals. Switching speed for these electrochromic devices is usually pretty slow though, on the magnitude of several minutes, so it's not likely to be instant darkening. The good news is that they switch at pretty low voltage