T&T: How does a "moisture meter" work?

[Bob McLeran]

Sorry to post this again, but I used the wrong term in the subject line 
and it may have resulted in some of our knowledgeable T&T Listees not 
responding.

Have a question about how a "moisture meter" (those seemingly magical
devices used to detect moisture in boat decks and elsewhere) actually
works. Do they really detect moisture, or do they simply sense the
density of the material? Is there a way to differentiate a change in
density caused by moisture from a change caused by other materials of
varying density in the structure?

-- 
<><><><><><><><><><><><>Mozilla Thunderbird<><><><><><><><><><>
Bob McLeran and Judy Young                  Manatee Cove Marina
M/V Walkabout (soon to be "Sanderling")     Patrick Air Force Base
Defever 41 Trawler                          Melbourne, Florida