|MadSci Network: Chemistry|
I wish there were an easy answer for your question, but I just don't know of one. The only material that comes to mind that visually changes in response to an increase in moisture is a sponge. However a sponge generally only swells in response to liquid; not necessarily an increase in water vapor. And a sponge will dry out and shrink even in humid conditions.
If you are really serious about finding a material with these properties, I would suggest contacting a local university with an engineering department.
Sorry I can't provide more information on this one.
Frederick James adds the following:
I have read that Hennig Briand, the (reputed) discoverer of phosphorus once traded his discovery for a toy consisting of a house from which a model man holding an umbrella would emerge in damp conditions. The device was actuated by a single strand of human hair, and further reading showed that the human hair stretches in an amount proportional to the humidity of an area. this can be used to make a moisture meter by attaching it to a first class lever as the effort, and having a load just sufficient to counterbalance the weight of a long indicator rod and provide a slight tension. the indicator rod need only be a long twig plased on the same side as the hair, and the hair anchored below the rod, so that when the hair is long the needle rises and the hair is short the needle drops. the hair actually is a pretty accurate measure of humidity, and i believe that it would be quite sufficient as the regulator for this persons intent.
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