I’m relatively ignorant on the subject but as I recall one of the practices of Scientology invovles some meter that tells you if you are “clear”. (Or something like that – I’m not meaning to open up a GD). My basic question is: Presumably, one of these has been grabbed by a non-believer or former Scientologist and been analyzed. What is it? What do non-Scientologist scientists/engineers say it measures and how?
You beat me to it, Friedo. 
Same link I was going to post.
That is unbelievable. I knew they were screwy, but…
It’s called an e-meter. It’s basically an amplified galvanometer. In reality, it measures skin resistance, and works as a primitive “lie detector”. Scientology claims it measures “mental charge”.
Some ex-Scientologists claim that the e-meter is used to deliver “subliminal electroshock” but there is no scientific support for this claim.
Like an ordinary polygraph, the “E-meter” simply measures galvanic skin resistance, which is sometimes referred to as the “sweat-index”. The more you perspire, the less electrical resistance your skin has. Water, as we all know, is a good conductor.
Wanna screw with your auditor and convince him/her you are rid of your Thetans? Show up for your session really hung-over and sweaty. Or, perhaps bone-dry from taking decongestants. Loads of laffs.
And if you want a really cheap E-meter, Radio Shack sells a “Biofeedback” machine that clamps on your finger and gives an audible tone corresponding to your sweatiness.
Buck The Diver<—proud owner of the RONCO Pocket E-Meter. Thetan-free since '83.
Cecil discusses Scientology and E-meters here: http://www.straightdope.com/classics/a4_134.html