What a Polygraph Is
First of all, what a polygraph IS NOT. A polygraph IS NOT a lie detector. There is no instrument on the planet that can detect lies.
What a polygraph instrument does is measure and record significant INVOLUNTARY responses or physiological changes such as blood pressure, heart rate, respiration and skin conductance when someone is under stress such as the stress induced when someone is attempting to deceive, i.e., is lying.
These responses are completely INVOLUNTARY, the examinee cannot control them. By analyzing the tracings on a polygraph chart an examiner can determine if the responses are indicative of deception.