Galvanic Skin Response as a measure of "engagement"

And of course, no one would think of manipulating this measurment scheme, by say, using outrageous materials or tactics in the classroom? By the way, why pay big buck for this, when you can make your own for 25 bucks (less if you don't use the kit)

http://dianeravitch.net/2012/06/10/a-flaw-in-the-galvanic-response-skin-bracelet/
http://dianeravitch.net/2012/06/10/this-teacher-knows-how-to-game-the-grs-bracelet/ 
http://dianeravitch.net/2012/06/18/is-it-hysterical-to-worry-about-those-gsr-monitors/

Galvanic Skin Response Kit

and a response/defense" http://www.forbes.com/sites/luisakroll/2012/06/13/gates-foundation-responds-to-gsr-bracelets-controversy/