The device is called a conscious. Almost everyone is taught from childhood that it is wrong to lie, so like it or not your brain and thus body react to telling a lie. You know that you are doing something wrong and thus it makes you nervous and uneasy. That reaction is what is detected, although it can sometimes be difficult to tell if you are nervous about the process, something else, or if you are just lying. That is why it takes a trained person to interpret the results and even then they can get it wrong which is why a lie detector is not considered to be valid evidence in court.
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Lie detectors do not catch someone telling a lie. The person that administers a polygraph test records the person's body responses each time the person is asked a question. People can fool a lie detector by clenching the anal sphincter whenever he or she is asked a neutral question. Thus the results become inconclusive. A better lie detector is a brain scan. The areas of the brain dealing with creative thoughts are very active whenever someone is lying. The problem is that it is too expensive to use the latter method.
Dear itsrkm, Different lie detectors use different methods but the polygraph, which is the most well know, measures electrical skin resistance using a Galvanic Skin response sensor. Apparently they are easy to make; here is a link to a site:http://www.extremenxt.com/gsr.htm I might be lying though. FOH
its called a suspicious husband.ha just kidding its called a polygraph