On what principle do electric alcohol detectors work?
I believe you are referring to breathalyzers. When you breathe into a breathalyzer, any ethanol present in your breath is oxidized to acetic acid (the main component of vinegar apart from water) at the anode (there is a diagram below).  At the cathode atmospheric oxygen is reduced. The overall reaction is the oxidation of ethanol to acetic acid and water. The electric current produced by this reaction is measured by a microprocessor and displayed as an approximation of overall blood alcohol content (BAC). 



This website can give you more information.

Hope this helps!